fix(encoding/form): FieldMask are converted to/from lower-camel naming conventions.

[tonybase]

// The JSON representation for a FieldMask is a JSON string where paths are
// separated by a comma. Fields name in each path are converted to/from
// lower-camel naming conventions. Encoding should fail if the path name would
// end up differently after a round-trip.

references: https://github.com/protocolbuffers/protobuf-go/blob/master/encoding/protojson/well_known_types.go#L842

[codecov-commenter]

Codecov Report

Merging #1724 (a1f4b1d) into main (b6b9508) will decrease coverage by 0.05%.
The diff coverage is 76.47%.

@@            Coverage Diff             @@
##             main    #1724      +/-   ##
==========================================
- Coverage   81.20%   81.14%   -0.06%     
==========================================
  Files          78       78              
  Lines        3442     3469      +27     
==========================================
+ Hits         2795     2815      +20     
- Misses        444      449       +5     
- Partials      203      205       +2     

Impacted Files	Coverage Δ
encoding/form/form.go	67.56% <66.66%> (ø)
encoding/form/proto_encode.go	72.00% <75.00%> (+0.18%)	⬆️
encoding/form/proto_decode.go	53.88% <78.57%> (+1.22%)	⬆️
transport/http/binding/encode.go	79.48% <100.00%> (ø)

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[shenqidebaozi]

LGTM