Fixing API (thanks Craig for the comments) and associated tests.

include/grpc/grpc_security.h

src/core/security/security_context.h

src/core/security/server_auth_filter.c

test/core/end2end/end2end_tests.h

test/core/end2end/fixtures/chttp2_fake_security.c

test/core/end2end/fixtures/chttp2_simple_ssl_fullstack.c

.../end2end/fixtures/chttp2_simple_ssl_fullstack_with_poll.c

...nd2end/fixtures/chttp2_simple_ssl_with_oauth2_fullstack.c

...2end/tests/request_response_with_payload_and_call_creds.c