Expose optional keyword for proto3

Hi @mafintosh - `optional` in proto3 has the meaning that the absence of the field can be distinguished from its default value. At the moment this module is swallowing all occurrences of the optional keyword so that users cannot see when fields have been marked this way.

I would suggest adding an `optional` boolean beside `required` and `repeated`. `required` would still always be false for proto3.

Would you accept a PR with this change ?

The issue does not name a file, entry point, or test. Start by locating the schema parser's handling of proto3 field modifiers and the existing required/repeated field metadata, then inspect related tests. Done means proto3 fields marked optional expose an optional boolean while required remains false.

Written by the indexing model from the issue text.