proto: Add `intercepted_method` extension to `GetObjectPool()` RPC
The intercepted_method
gRPC extension is used to denote that an RPC is
intercepted by Praefect and not transparently proxied. In 2c8e1902, a
stream handler for the GetObjectPool()
RPC was introducted in Praefect
and used to intercept the RPC.
Add the intercepted_method
extension to the GetObjectPool
method
protobuf definition and remove the op_type
extension because it is not
required by intercepted methods.