Setting the newly added compression_level field of
grpc_op::send_initial_metadata by a server now has the effect of
applying that compression level for the subsequent call messages leaving
the server. The ultimate meaning of the level depends on the client's
supported compression algorithms.

application handle the authentication.

The purpose of this is to be able to install a composition policy that
describes which types are incompatible and that will be enforced during
call creds composition.

If this functionality is wanted it will be done in an additive function in
the API like :

void grpc_call_credentials_set_composite_policy(
  grpc_call_credentials_composite_policy policy);