Support unknown json data and add internal set/clear field methods for generated accessors. #1324

Job	Run time
format_analyze; linux; Dart dev; PKG: benchmarks; `dart format --output=none --set-exit-if-change...	21s
format_analyze; linux; Dart 2.19.0; PKG: protobuf; `dart analyze lib`, `dart analyze test`	19s
mono_repo self validate	16s
format_analyze; linux; Dart dev; PKG: api_benchmark; `dart format --output=none --set-exit-if-cha...	27s
format_analyze; linux; Dart dev; PKG: protobuf; `dart format --output=none --set-exit-if-changed ...	22s
format_analyze; linux; Dart dev; PKG: protoc_plugin; `dart format --output=none --set-exit-if-cha...	30s
run_tests; linux; Dart dev; PKG: protobuf; `dart test`	24s
run_tests; linux; Dart 2.19.0; PKG: protobuf; `dart test`	29s
run_tests; linux; Dart 2.19.0; PKG: protoc_plugin; `./../tool/setup.sh`, `make protos`, `dart test`	48s
run_tests; osx; Dart 2.19.0; PKG: protobuf; `dart test`	55s
run_tests; linux; Dart dev; PKG: protoc_plugin; `./../tool/setup.sh`, `make protos`, `dart test`	32s
run_tests; osx; Dart dev; PKG: protobuf; `dart test`	44s
run_tests; windows; Dart 2.19.0; PKG: protobuf; `dart test`	42s
run_tests; windows; Dart dev; PKG: protobuf; `dart test`	37s
run_legacy_tests; linux; Dart 2.19.0; PKG: protoc_plugin; `./../tool/setup.sh`, `make protos`, `d...	0s
	7m 26s

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