Add support for Protobuf options / extensions

### Clear and concise description of the problem

Currently it appears as though the standard Protobuf 3 language feature of (custom) options / extensions is not supported for [decoration](https://typespec.io/docs/libraries/protobuf/reference#decorators) and thus emission:
- https://protobuf.dev/programming-guides/proto3/#customoptions

In many practical examples, extensions are used to add additional metadata to the Protobuf IDL.

### Checklist

- [X] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [X] Read the [docs](https://typespec.io/docs/).
- [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.