feat: Serve gRPC and REST requests on the same external port

[vchudnov-g]

This PR uses the cmux package to expose a single external port that selects incoming connections to route to the gRPC endpoint or the REST endpoint.
(ref: https://medium.com/@drgarcia1986/listen-grpc-and-http-requests-on-the-same-port-263c40cb45ff and https://github.com/soheilhy/cmux)

I define an Endpoint interface wrapping both the gRPC and the REST endpoints, as well as the cmux multiplexer that does the redirection. This infrastructure will also allow future endpoints to be easily added when we need them.

This also sets up the infrastructure for graceful shutdown upon receipt of a signal, though we are waiting for a PR in the cmux dependency for that to work fully. (The issue is described in soheilhy/cmux#69 (comment).)

Implementation/review notes:

• the gRPC server set-up code is now in cmd/gapic-showcase/endpoint.go. It is split between setting up the server and running it.
• the REST server (also in endpoint.go) is currently a stub that simply responds to /hello requests. It will be expanded with auto-generated handlers.

[codecov]

Codecov Report

Merging #489 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #489   +/-   ##
=======================================
  Coverage   97.81%   97.81%           
=======================================
  Files          14       14           
  Lines        1007     1007           
=======================================
  Hits          985      985           
  Misses         15       15           
  Partials        7        7           

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