[googleapis_auth] feat: Add ImpersonatedAuthClient for service account impersonation #1282

Job	Run time
analyze_and_format; Dart 3.9.0; PKGS: _test, _test_package, discoveryapis_commons, discoveryapis_...	2m 46s
analyze_and_format; Dart dev; PKGS: _test, _test_package, discoveryapis_commons, discoveryapis_ge...	3m 51s
mono_repo self validate	23s
unittest; Dart dev; PKG: discoveryapis_generator; `dart test`	40s
unittest; Dart 3.9.0; PKG: googleapis_auth; `dart test -p chrome --compiler dart2js,dart2wasm`	1m 34s
unittest; Dart dev; PKG: discoveryapis_commons; `dart test`	41s
unittest; Dart dev; PKG: _test_package; `dart test`	36s
unittest; Dart 3.9.0; PKG: generated/googleapis_beta; `dart test`	1m 15s
unittest; Dart dev; PKG: generated/googleapis_beta; `dart test`	1m 25s
unittest; Dart dev; PKG: googleapis_auth; `dart test -p vm`	36s
unittest; Dart 3.9.0; PKG: discoveryapis_generator; `dart test`	42s
unittest; Dart 3.9.0; PKG: generated/googleapis; `dart test`	2m 43s
unittest; Dart 3.9.0; PKG: _test_package; `dart test`	25s
unittest; Dart 3.9.0; PKG: discoveryapis_commons; `dart test`	46s
unittest; Dart dev; PKG: generated/googleapis; `dart test`	2m 36s
unittest; Dart 3.9.0; PKG: googleapis_auth; `dart test -p vm`	26s
unittest; Dart dev; PKG: googleapis_auth; `dart test -p chrome --compiler dart2js,dart2wasm`	1m 42s
	23m 7s

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