[googleapis_auth] feat: Add ImpersonatedAuthClient for service account impersonation #1277

Job	Run time
analyze_and_format; Dart dev; PKGS: _test, _test_package, discoveryapis_commons, discoveryapis_ge...	3m 44s
mono_repo self validate	24s
analyze_and_format; Dart 3.9.0; PKGS: _test, _test_package, discoveryapis_commons, discoveryapis_...	2m 9s
unittest; Dart 3.9.0; PKG: generated/googleapis_beta; `dart test`	46s
unittest; Dart 3.9.0; PKG: discoveryapis_commons; `dart test`	34s
unittest; Dart 3.9.0; PKG: _test_package; `dart test`	29s
unittest; Dart dev; PKG: generated/googleapis_beta; `dart test`	31s
unittest; Dart dev; PKG: googleapis_auth; `dart test -p chrome --compiler dart2js,dart2wasm`	1m 19s
unittest; Dart dev; PKG: googleapis_auth; `dart test -p vm`	40s
unittest; Dart 3.9.0; PKG: generated/googleapis; `dart test`	2m 16s
unittest; Dart 3.9.0; PKG: googleapis_auth; `dart test -p chrome --compiler dart2js,dart2wasm`	1m 39s
unittest; Dart dev; PKG: generated/googleapis; `dart test`	2m 15s
unittest; Dart 3.9.0; PKG: googleapis_auth; `dart test -p vm`	26s
unittest; Dart dev; PKG: discoveryapis_generator; `dart test`	29s
unittest; Dart dev; PKG: _test_package; `dart test`	27s
unittest; Dart 3.9.0; PKG: discoveryapis_generator; `dart test`	32s
unittest; Dart dev; PKG: discoveryapis_commons; `dart test`	27s
	19m 7s

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