Need support for idempotency level in method options

[arvindram03]

Looks like the protobuf/lib/protobuf/descriptors/google/protobuf/descriptor.pb.rb has not been updated for the past three years.

This does not include the idempotency_level field in the MethodOptions class.

It is present in the google protobuf method descriptor.
https://github.com/protocolbuffers/protobuf/blob/master/src/google/protobuf/descriptor.proto#L710

Can you please update the descriptor to support this field ?