Extensions RPC lifetime mismatch between server and JobHost leads to incorrect RPC calls

The current design of the extension RPC server has it piggy backing off the general worker gRPC server. The lifetime mismatch between individual workers and this server causes issues during job host transitions. We will need explore one of two options:

1. Improve lifetime tracking with the existing RPC server
- Only unregister endpoints when functions have finished draining.
- Support multiple job hosts being registered. Will need to include an identifier for what set of endpoints an extension call needs to be routed to.
2. OR move these endpoints into a new server that lives in the job host.
- Will need to measure the impact an extra server startup will have.