Add option to enable dynamic routing in Envoy Proxy

[kondratyevd]

One can now set the following parameters in values.yaml:

envoy:
  dynamic_routing:
    enabled: true
  lua_filter:
    enabled: true
    lua_config: "cfg/envoy-filter-dynamic.lua"

In envoy-filter-dynamic.lua, we add logic to inject an HTTP header the value of which contains the address to which the request will be routed by Envoy.

• For inference requests, we extract model name from request body and redirect the request to model-specific load balancer.
• For model repository index requests, we will redirect the request to a repository index aggregator.

[kondratyevd]

This repo has a prototype python framework to dynamically load/unload models to different Triton servers: https://github.com/kondratyevd/supersonic-model-loader