[googleapis_auth] feat: Add sign() method to ServiceAccountCredentials #1280

Job	Run time
analyze_and_format; Dart 3.9.0; PKGS: _test, _test_package, discoveryapis_commons, discoveryapis_...	2m 13s
mono_repo self validate	34s
analyze_and_format; Dart dev; PKGS: _test, _test_package, discoveryapis_commons, discoveryapis_ge...	4m 4s
unittest; Dart dev; PKG: discoveryapis_commons; `dart test`	33s
unittest; Dart 3.9.0; PKG: _test_package; `dart test`	39s
unittest; Dart 3.9.0; PKG: googleapis_auth; `dart test -p chrome --compiler dart2js,dart2wasm`	1m 27s
unittest; Dart 3.9.0; PKG: generated/googleapis; `dart test`	2m 22s
unittest; Dart dev; PKG: discoveryapis_generator; `dart test`	41s
unittest; Dart dev; PKG: googleapis_auth; `dart test -p vm`	30s
unittest; Dart dev; PKG: googleapis_auth; `dart test -p chrome --compiler dart2js,dart2wasm`	1m 31s
unittest; Dart dev; PKG: generated/googleapis; `dart test`	2m 28s
unittest; Dart 3.9.0; PKG: googleapis_auth; `dart test -p vm`	28s
unittest; Dart 3.9.0; PKG: discoveryapis_commons; `dart test`	41s
unittest; Dart dev; PKG: _test_package; `dart test`	22s
unittest; Dart dev; PKG: generated/googleapis_beta; `dart test`	38s
unittest; Dart 3.9.0; PKG: discoveryapis_generator; `dart test`	44s
unittest; Dart 3.9.0; PKG: generated/googleapis_beta; `dart test`	40s
	20m 35s

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