Use dynamic channel selection to reduce server code duplication

[snowp]

Currently the server code manages a set of typed channels/cancelations/etc. and an untyped aggregation goroutine that allows for handling generic types. This is done because of how select only supports a static number of channels. As a result of this, there is a lot of duplicated code between the typed branches and a different set of logic handling the unknown types.

We might consider using reflect.Select here in order to treat all the channels the same: this would not only reduce the amount of code duplication, but also reduce the number of goroutines used when unknown types are subscribed to.

https://github.com/envoyproxy/go-control-plane/compare/main...snowp:dynamic-select?expand=1 shows a prototype of reflect.Select in action: we manage a list of channels to subscribe from based on, and use reflect.Select to pick between the watch channels, the request channel and ctx.Done.

Putting this up for thoughts in case I'm missing any cons with the reflection approach here.

[alecholmez]

I just opened this: #438

It's not as elegant as the dynamic channel selection but it reduces the code duplication we have in SOTW.