[googleapis_auth] feat: Add ImpersonatedAuthClient for service account impersonation #1296

Job	Run time
analyze_and_format; Dart dev; PKGS: _test, _test_package, discoveryapis_commons, discoveryapis_ge...	3m 44s
mono_repo self validate	31s
analyze_and_format; Dart 3.9.0; PKGS: _test, _test_package, discoveryapis_commons, discoveryapis_...	2m 22s
unittest; Dart 3.9.0; PKG: googleapis_auth; `dart test -p chrome --compiler dart2js,dart2wasm`	1m 38s
unittest; Dart dev; PKG: _test_package; `dart test`	25s
unittest; Dart 3.9.0; PKG: _test_package; `dart test`	26s
unittest; Dart dev; PKG: googleapis_auth; `dart test -p chrome --compiler dart2js,dart2wasm`	1m 26s
unittest; Dart dev; PKG: googleapis_auth; `dart test -p vm`	25s
unittest; Dart 3.9.0; PKG: discoveryapis_generator; `dart test`	34s
unittest; Dart 3.9.0; PKG: generated/googleapis; `dart test`	2m 30s
unittest; Dart dev; PKG: discoveryapis_generator; `dart test`	37s
unittest; Dart dev; PKG: generated/googleapis; `dart test`	2m 38s
unittest; Dart 3.9.0; PKG: googleapis_auth; `dart test -p vm`	28s
unittest; Dart dev; PKG: generated/googleapis_beta; `dart test`	46s
unittest; Dart 3.9.0; PKG: discoveryapis_commons; `dart test`	28s
unittest; Dart 3.9.0; PKG: generated/googleapis_beta; `dart test`	36s
unittest; Dart dev; PKG: discoveryapis_commons; `dart test`	41s
	20m 15s

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