Add `river validate` subcommand

[dhermes]

Reading https://riverqueue.com/docs/migrations I'm struck by the fact that it may be hard (over time) to figure out if the version of the schema I'm running in my database matches the schema expected by the version river that I've got in my go.mod / go.sum.

I also think having river validate would make it much easier for teams like mine to integrate this into our existing migrations workflow. I.e. we could run river validate in CI and during development and detect if our existing migrations sequence needs to be updated.

(I don't think it'd be the end of the world to add river migrate-up during deploy right next to our migrate up command. However, having the two work together during development is preferable to having two "migrations managers" running during our deploys.)

[brandur]

This is a good idea. I'm going to be refactoring the migrations component so it can be a Go API, and I'll take a look at this after.

We're going to start tracking feature requests as GitHub discussions, so I'm going to convert this issue to one.

Anyone finding this: use the upvote reactji if you'd find this useful to help us prioritize.