Add bg task for collecting chicken switches from DB

[andrewjstone]

Wire up the task watcher into the planner so that it only runs when the planner_enabled switch is set to `true.

Fixes #8253

[andrewjstone]

Manual testing:

➜  omicron git:(ajs/chicken-switch-bg-task) cargo run -p omicron-omdb nexus --nexus-internal-url='http://[::1]:12221' chicken-switches show current
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.87s
     Running `target/debug/omdb nexus '--nexus-internal-url=http://[::1]:12221' chicken-switches show current`
note: using Nexus URL http://[::1]:12221
No chicken switches enabled

➜  omicron git:(ajs/chicken-switch-bg-task) cargo run -p omicron-omdb nexus --nexus-internal-url='http://[::1]:12221' background-tasks show blueprint_planner
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.78s
     Running `target/debug/omdb nexus '--nexus-internal-url=http://[::1]:12221' background-tasks show blueprint_planner`
note: using Nexus URL http://[::1]:12221
task: "blueprint_planner"
  configured period: every 1m
  currently executing: no
  last completed activation: iter 8, triggered by a periodic timer firing
    started at 2025-06-26T21:23:41.949Z (16s ago) and ran for 0ms
    blueprint planning explicitly disabled by config!
    
➜  omicron git:(ajs/chicken-switch-bg-task) cargo run -p omicron-omdb nexus --nexus-internal-url='http://[::1]:12221' chicken-switches set --planner-enabled true -w
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.86s
     Running `target/debug/omdb nexus '--nexus-internal-url=http://[::1]:12221' chicken-switches set --planner-enabled true -w`
note: using Nexus URL http://[::1]:12221
Chicken switches updated at version 1

➜  omicron git:(ajs/chicken-switch-bg-task) cargo run -p omicron-omdb nexus --nexus-internal-url='http://[::1]:12221' chicken-switches show current
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.78s
     Running `target/debug/omdb nexus '--nexus-internal-url=http://[::1]:12221' chicken-switches show current`
note: using Nexus URL http://[::1]:12221
Reconfigurator Chicken Switches:
    version: 1
    modified time: 2025-06-26 21:24:31.546880 UTC
    planner enabled: true

➜  omicron git:(ajs/chicken-switch-bg-task) cargo run -p omicron-omdb nexus --nexus-internal-url='http://[::1]:12221' background-tasks show blueprint_planner
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.81s
     Running `target/debug/omdb nexus '--nexus-internal-url=http://[::1]:12221' background-tasks show blueprint_planner`
note: using Nexus URL http://[::1]:12221
task: "blueprint_planner"
  configured period: every 1m
  currently executing: no
  last completed activation: iter 11, triggered by a dependent task completing
    started at 2025-06-26T21:24:42.200Z (23s ago) and ran for 99ms
    plan unchanged from parent 0a6670a1-ccf9-4062-be6d-5f6e6821e644
    
➜  omicron git:(ajs/chicken-switch-bg-task) cargo run -p omicron-omdb nexus --nexus-internal-url='http://[::1]:12221' chicken-switches set --planner-enabled false -w
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.82s
     Running `target/debug/omdb nexus '--nexus-internal-url=http://[::1]:12221' chicken-switches set --planner-enabled false -w`
note: using Nexus URL http://[::1]:12221
Chicken switches updated at version 2

➜  omicron git:(ajs/chicken-switch-bg-task) cargo run -p omicron-omdb nexus --nexus-internal-url='http://[::1]:12221' chicken-switches show current
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.80s
     Running `target/debug/omdb nexus '--nexus-internal-url=http://[::1]:12221' chicken-switches show current`
note: using Nexus URL http://[::1]:12221
Reconfigurator Chicken Switches:
    version: 2
    modified time: 2025-06-26 21:25:38.912297 UTC
    planner enabled: false

➜  omicron git:(ajs/chicken-switch-bg-task) cargo run -p omicron-omdb nexus --nexus-internal-url='http://[::1]:12221' background-tasks show blueprint_planner
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.94s
     Running `target/debug/omdb nexus '--nexus-internal-url=http://[::1]:12221' background-tasks show blueprint_planner`
note: using Nexus URL http://[::1]:12221
task: "blueprint_planner"
  configured period: every 1m
  currently executing: no
  last completed activation: iter 13, triggered by a dependent task completing
    started at 2025-06-26T21:25:42.178Z (0s ago) and ran for 0ms
    blueprint planning explicitly disabled by config!
    
➜  omicron git:(ajs/chicken-switch-bg-task) cargo run -p omicron-omdb nexus --nexus-internal-url='http://[::1]:12221' chicken-switches show 1
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.78s
     Running `target/debug/omdb nexus '--nexus-internal-url=http://[::1]:12221' chicken-switches show 1`
note: using Nexus URL http://[::1]:12221
Reconfigurator Chicken Switches:
    version: 1
    modified time: 2025-06-26 21:24:31.546880 UTC
    planner enabled: true

[jgallagher]

Looks great, just some minor nits 👍

[andrewjstone]

I followed the same testing as above after implementing the review fixes and everything works.