feat(context): add Protocol Buffers support to content negotiation

[1911860538]

This change allows API clients to receive Protocol Buffers encoded responses,
via standard content negotiation when using the Context.Negotiate method.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.96%. Comparing base (3dc1cd6) to head (bde594c).
⚠️ Report is 201 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4423      +/-   ##
==========================================
- Coverage   99.21%   98.96%   -0.25%     
==========================================
  Files          42       44       +2     
  Lines        3182     2902     -280     
==========================================
- Hits         3157     2872     -285     
- Misses         17       19       +2     
- Partials        8       11       +3     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.95% <100.00%> (?)
-tags go_json	98.88% <100.00%> (?)
-tags nomsgpack	98.95% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.24	98.96% <100.00%> (?)
go-1.25	98.96% <100.00%> (?)
macos-latest	98.96% <100.00%> (-0.25%)	⬇️
ubuntu-latest	98.96% <100.00%> (-0.25%)	⬇️

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