[googleapis_auth] feat: Add ImpersonatedAuthClient for service accoun… #1297

Job	Run time
analyze_and_format; Dart dev; PKGS: _test, _test_package, discoveryapis_commons, discoveryapis_ge...	3m 53s
mono_repo self validate	32s
analyze_and_format; Dart 3.9.0; PKGS: _test, _test_package, discoveryapis_commons, discoveryapis_...	2m 18s
unittest; Dart 3.9.0; PKG: generated/googleapis_beta; `dart test`	50s
unittest; Dart 3.9.0; PKG: _test_package; `dart test`	43s
unittest; Dart dev; PKG: googleapis_auth; `dart test -p vm`	41s
unittest; Dart 3.9.0; PKG: discoveryapis_generator; `dart test`	47s
unittest; Dart 3.9.0; PKG: googleapis_auth; `dart test -p vm`	45s
unittest; Dart dev; PKG: _test_package; `dart test`	35s
unittest; Dart 3.9.0; PKG: generated/googleapis; `dart test`	2m 47s
unittest; Dart dev; PKG: discoveryapis_commons; `dart test`	43s
unittest; Dart dev; PKG: generated/googleapis_beta; `dart test`	44s
unittest; Dart dev; PKG: generated/googleapis; `dart test`	2m 40s
unittest; Dart dev; PKG: discoveryapis_generator; `dart test`	41s
unittest; Dart dev; PKG: googleapis_auth; `dart test -p chrome --compiler dart2js,dart2wasm`	1m 27s
unittest; Dart 3.9.0; PKG: discoveryapis_commons; `dart test`	44s
unittest; Dart 3.9.0; PKG: googleapis_auth; `dart test -p chrome --compiler dart2js,dart2wasm`	1m 38s
	22m 28s

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