The gist here is that now we use a value-type struct proxycfg.UpstreamID
as the map key in ConfigSnapshot maps where we used to use "upstream
id-ish" strings. These are internal only and used just for bidirectional
trips through the agent cache keyspace (like the discovery chain target
struct).
For the few places where the upstream id needs to be projected into xDS,
that's what (proxycfg.UpstreamID).EnvoyID() is for. This lets us ALWAYS
inject the partition and namespace into these things without making
stuff like the golden testdata diverge.
These methods only called a single function. Wrappers like this end up making code harder to read
because it adds extra ways of doing things.
We already have many helper functions for constructing these types, we don't need additional methods.
There is no interaction between these handlers, so splitting them into separate files
makes it easier to discover the full implementation of each kindHandler.
