Move signBlob to a function, remove IAMSigner class #1323

Job	Run time
analyze_and_format; Dart dev; PKGS: _test, _test_package, discoveryapis_commons, discoveryapis_ge...	3m 55s
mono_repo self validate	20s
analyze_and_format; Dart 3.9.0; PKGS: _test, _test_package, discoveryapis_commons, discoveryapis_...	2m 12s
unittest; Dart dev; PKG: generated/googleapis_beta; `dart test`	45s
unittest; Dart 3.9.0; PKG: googleapis_auth; `dart test -p chrome --compiler dart2js,dart2wasm`	2m 7s
unittest; Dart 3.9.0; PKG: generated/googleapis; `dart test`	2m 43s
unittest; Dart 3.9.0; PKG: discoveryapis_generator; `dart test`	47s
unittest; Dart dev; PKG: generated/googleapis; `dart test`	2m 42s
unittest; Dart 3.9.0; PKG: discoveryapis_commons; `dart test`	31s
unittest; Dart dev; PKG: discoveryapis_commons; `dart test`	39s
unittest; Dart dev; PKG: googleapis_auth; `dart test -p vm`	42s
unittest; Dart dev; PKG: googleapis_auth; `dart test -p chrome --compiler dart2js,dart2wasm`	1m 42s
unittest; Dart 3.9.0; PKG: googleapis_auth; `dart test -p vm`	42s
unittest; Dart 3.9.0; PKG: _test_package; `dart test`	41s
unittest; Dart dev; PKG: _test_package; `dart test`	25s
unittest; Dart 3.9.0; PKG: generated/googleapis_beta; `dart test`	49s
unittest; Dart dev; PKG: discoveryapis_generator; `dart test`	31s
	22m 13s

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