channelz: refactor to move proto API to a separate package

[dfawley]

I am very very sorry @Clement-Jean - this change will conflict heavily with #7059, but is needed to have a stable API (for an external dependency) that doesn't require the channelz service itself. I can help resolve the conflicts it creates if it's too much work.

RELEASE NOTES: none

[codecov]

Codecov Report

Merging #7065 (1c8b93e) into master (b78c0eb) will decrease coverage by 0.12%.
The diff coverage is 82.94%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7065      +/-   ##
==========================================
- Coverage   82.53%   82.41%   -0.12%     
==========================================
  Files         300      305       +5     
  Lines       31357    31383      +26     
==========================================
- Hits        25881    25865      -16     
- Misses       4425     4453      +28     
- Partials     1051     1065      +14     

Files	Coverage Δ
channelz/internal/protoconv/sockopt_linux.go	85.88% <ø> (ø)
channelz/internal/protoconv/util.go	100.00% <100.00%> (ø)
channelz/internal/protoconv/subchannel.go	89.28% <89.28%> (ø)
channelz/internal/protoconv/server.go	71.87% <71.87%> (ø)
channelz/internal/protoconv/socket.go	89.15% <89.15%> (ø)
channelz/service/service.go	64.28% <54.16%> (-22.12%)	⬇️
channelz/internal/protoconv/channel.go	86.04% <86.04%> (ø)

... and 19 files with indirect coverage changes

[Clement-Jean]

@dfawley No worries, I'll try to resolve them first.