Support proto3 "presence" feature

[ghaskins]

https://github.com/protocolbuffers/protobuf/blob/v3.12.0-rc1/docs/implementing_proto3_presence.md

[spencerwilson]

Since this issue was created, Protobuf 3.15.0 was released. Relevant changes:

• 3.15.0 added a supported_features field to the google.protobuf.compiler.CodeGeneratorResponse message: https://github.com/protocolbuffers/protobuf/blob/v24.4/src/google/protobuf/compiler/plugin.proto#L117-L126. Plugins can use this to report post hoc which features they supported.
• https://github.com/protocolbuffers/protobuf/blob/v24.4/docs/field_presence.md was created, describing how to use field presence and noting that it's on by default in compiler 3.15.0+
• The doc linked in OP is still around, but may have changed in the last few years: https://github.com/protocolbuffers/protobuf/blob/v24.4/docs/implementing_proto3_presence.md

My attention was drawn to this issue when I was struggling to access an optional field in emitted Clojure code:

  optional bool dryRun = 8;

was becoming

(= (get-in req [:grpc-params :-dryRun])
   {:dryRun false})

rather than the expected

(= (get-in req [:grpc-params :dryRun]) false)

(h/t to @sundbry, who explained: This is a consequence of the behavior described in implementing_proto3_presence.md: "When a user adds an optional field to proto3, this is internally rewritten as a one-field oneof, for backward-compatibility with reflection-based algorithms" The leading - is an accident of it being treated like oneof.)

This also comes through in the output of buf generate using the plugin in this repo:

% buf generate
Warning: plugin "clojure-wrapper" does not support required features.
  Feature "proto3 optional" is required by 1 file(s):
    my_file.proto

Among the work needed for this issue is upgrading the version of Google's plugin.proto to a more recent definition, one that includes the supported_features field.

In the meantime, I'll code against the synthetic oneof representation currently being generated by the plugin.

[ghaskins]

@spencerwilson I took a crack at the first part: protojure/google.protobuf#9