Support optional baseClient in ImpersonatedAuthClient, and safely ext… #1351

Job	Run time
analyze_and_format; Dart dev; PKGS: _test, _test_package, discoveryapis_commons, discoveryapis_ge...	3m 52s
mono_repo self validate	31s
analyze_and_format; Dart 3.9.0; PKGS: _test, _test_package, discoveryapis_commons, discoveryapis_...	2m 16s
unittest; Dart 3.9.0; PKG: _test_package; `dart test`	40s
unittest; Dart 3.9.0; PKG: discoveryapis_commons; `dart test`	42s
unittest; Dart 3.9.0; PKG: googleapis_auth; `dart test -p chrome --compiler dart2js,dart2wasm`	1m 45s
unittest; Dart 3.9.0; PKG: generated/googleapis; `dart test`	2m 45s
unittest; Dart 3.9.0; PKG: generated/googleapis_beta; `dart test`	50s
unittest; Dart dev; PKG: generated/googleapis; `dart test`	2m 45s
unittest; Dart dev; PKG: generated/googleapis_beta; `dart test`	36s
unittest; Dart 3.9.0; PKG: googleapis_auth; `dart test -p vm`	35s
unittest; Dart dev; PKG: _test_package; `dart test`	38s
unittest; Dart dev; PKG: googleapis_auth; `dart test -p vm`	39s
unittest; Dart dev; PKG: discoveryapis_generator; `dart test`	43s
unittest; Dart 3.9.0; PKG: discoveryapis_generator; `dart test`	36s
unittest; Dart dev; PKG: googleapis_auth; `dart test -p chrome --compiler dart2js,dart2wasm`	1m 46s
unittest; Dart dev; PKG: discoveryapis_commons; `dart test`	29s
analyze_format; Dart 3.9.0; PKG: _googleapis_auth_e2e; `dart analyze`	25s
analyze_format; Dart dev; PKG: _googleapis_auth_e2e; `dart format --output=none --set-exit-if-cha...	24s
	22m 57s

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