Merge pull request #11733 from markdroth/security_api_cleanup

include/grpc/impl/codegen/atm.h

...ib/security/credentials/composite/composite_credentials.c

src/core/lib/security/credentials/credentials.c

src/core/lib/security/credentials/credentials.h

src/core/lib/security/credentials/credentials_metadata.c

src/core/lib/security/credentials/fake/fake_credentials.c

src/core/lib/security/credentials/fake/fake_credentials.h

src/core/lib/security/credentials/iam/iam_credentials.c

src/core/lib/security/credentials/iam/iam_credentials.h

src/core/lib/security/credentials/jwt/jwt_credentials.c

src/core/lib/security/credentials/jwt/jwt_credentials.h

...core/lib/security/credentials/oauth2/oauth2_credentials.c

...core/lib/security/credentials/oauth2/oauth2_credentials.h

...core/lib/security/credentials/plugin/plugin_credentials.c

...core/lib/security/credentials/plugin/plugin_credentials.h

src/core/lib/security/transport/client_auth_filter.c

src/core/lib/security/transport/security_connector.c

src/core/lib/security/transport/security_connector.h

src/node/test/credentials_test.js

test/core/end2end/fixtures/h2_oauth2.c