protoc-gen-ent: support proto3 optional fields (explicit field presence)

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## Summary 💡

About a year ago with release 3.15, the protoc maintainers enabled support for the old `optional` field modifier in proto3 by default, which is used to indicate field presence when the zero-value cannot be used. protoc-gen-ent does not yet support this.

## Motivation 🔦

Instead of writing `google.protobuf.StringValue myfield = 1;` one can again simply write `optional string myfield = 1;` when an empty string has special meaning. In Go this results in a string pointer field and a slightly changed getter method.