runtime: make HTTPBodyMarshaler the default

[johanbrandhorst]

google.api.HttpBody provides complete control over
the output format and content type, and we should
support this natively in the gateway, and not force
users to set their marshaler explicitly.

[codecov-commenter]

Codecov Report

Merging #1385 into v2 will increase coverage by 0.05%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##               v2    #1385      +/-   ##
==========================================
+ Coverage   58.81%   58.87%   +0.05%     
==========================================
  Files          32       32              
  Lines        3904     3895       -9     
==========================================
- Hits         2296     2293       -3     
+ Misses       1354     1349       -5     
+ Partials      254      253       -1     

Impacted Files	Coverage Δ
runtime/marshal_json.go	66.66% <0.00%> (ø)
runtime/marshal_proto.go	55.55% <0.00%> (ø)
runtime/marshaler.go	100.00% <ø> (ø)
runtime/marshaler_registry.go	85.71% <ø> (ø)
runtime/errors.go	48.48% <100.00%> (+1.42%)	⬆️
runtime/handler.go	45.29% <100.00%> (-0.08%)	⬇️
runtime/marshal_httpbodyproto.go	100.00% <100.00%> (+23.07%)	⬆️
runtime/marshal_jsonpb.go	67.74% <100.00%> (ø)

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