[googleapis_auth] feat: Add sign() method to ServiceAccountCredentials #1275

Job	Run time
analyze_and_format; Dart dev; PKGS: _test, _test_package, discoveryapis_commons, discoveryapis_ge...	3m 54s
analyze_and_format; Dart 3.9.0; PKGS: _test, _test_package, discoveryapis_commons, discoveryapis_...	2m 31s
mono_repo self validate	23s
unittest; Dart 3.9.0; PKG: discoveryapis_commons; `dart test`	41s
unittest; Dart 3.9.0; PKG: googleapis_auth; `dart test -p chrome --compiler dart2js,dart2wasm`	1m 46s
unittest; Dart dev; PKG: _test_package; `dart test`	23s
unittest; Dart 3.9.0; PKG: googleapis_auth; `dart test -p vm`	40s
unittest; Dart 3.9.0; PKG: discoveryapis_generator; `dart test`	43s
unittest; Dart 3.9.0; PKG: _test_package; `dart test`	38s
unittest; Dart 3.9.0; PKG: generated/googleapis_beta; `dart test`	47s
unittest; Dart 3.9.0; PKG: generated/googleapis; `dart test`	2m 35s
unittest; Dart dev; PKG: googleapis_auth; `dart test -p vm`	26s
unittest; Dart dev; PKG: discoveryapis_commons; `dart test`	26s
unittest; Dart dev; PKG: generated/googleapis_beta; `dart test`	39s
unittest; Dart dev; PKG: googleapis_auth; `dart test -p chrome --compiler dart2js,dart2wasm`	1m 20s
unittest; Dart dev; PKG: generated/googleapis; `dart test`	2m 36s
unittest; Dart dev; PKG: discoveryapis_generator; `dart test`	40s
	21m 8s

Provide feedback