add `enum_attribute` and `message_attribute` to `tonic-build`

[hawkw]

Feature Request

Crates

• tonic-build

Motivation

prost-build v0.11.6 added new enum_attribute and message_attribute methods to the builder for setting attributes on only enum (proto enums or oneofs) and message types (see tokio-rs/prost#784). However, tonic-build's builder currently only exposes the type_attribute and field_attribute methods from prost-build.

Proposal

We should add new builder methods corresponding to those prost-build methods.

Alternatives

Not do this?

[hawkw]

Oh, whoops, I see that this just merged in #1234. My bad!