Support custom response message

[goccy]

Spec: https://github.com/mercari/grpc-federation/blob/support-custom-response/docs/references.md#grpcfederationmethodresponse

[github-actions]

Code Metrics Report

	main (228a616)	#242 (da2aa5f)	+/-
Coverage	66.1%	66.3%	+0.1%
Code to Test Ratio	1:0.3	1:0.3	-0.0
Test Execution Time	7m8s	7m18s	+10s

Details

  |                     | main (228a616) | #242 (da2aa5f) |  +/-  |
  |---------------------|----------------|----------------|-------|
+ | Coverage            |          66.1% |          66.3% | +0.1% |
  |   Files             |             72 |             73 |    +1 |
  |   Lines             |          12183 |          12242 |   +59 |
+ |   Covered           |           8059 |           8111 |   +52 |
- | Code to Test Ratio  |          1:0.3 |          1:0.3 |  -0.0 |
  |   Code              |          37640 |          37918 |  +278 |
+ |   Test              |          12986 |          13075 |   +89 |
- | Test Execution Time |           7m8s |          7m18s |  +10s |

Code coverage of files in pull request scope (77.6% → 77.7%)

Files	Coverage	+/-
generator/code_generator.go	79.4%	+0.3%
grpc/federation/generator/decode.go	73.3%	-0.0%
grpc/federation/generator/encode.go	82.3%	+0.1%
internal/testutil/builder.go	84.5%	+0.1%
resolver/method.go	100.0%	+100.0%
resolver/resolver.go	83.4%	+0.1%
resolver/service.go	78.7%	0.0%
resolver/types.go	86.7%	0.0%
source/clone.go	75.8%	0.0%
source/file.go	65.1%	-0.2%
source/location.go	0.0%	0.0%
source/location_builder.go	68.5%	+0.3%

---

Reported by octocov

[utahta]

LGTM!