Protobuf codec and format support

[kconwayinvision]

Hey folks, I have an implementation of https://github.com/cloudevents/spec/blob/v1.0.1/protobuf-format.md that I'm looking to contribute. Before I open a PR, I figured I'd give folks a chance to see what is involved. I've put some code up for demonstration here: https://github.com/kconwayinvision/cloudevents-go-protobuf. It's a fairly straightforward implementation of format.Format, datacodec.Encoder, and datacodec.Decoder. The real complexity is in implementing the requirement to use the ProtoData field when the format and data codec are both protobuf.

If this belongs in the SDK then I can open a PR for us to talk about more specific issues such as whether or not to automate protobuf code generation or the complexity of the protobuf type juggling. I figured I'd start by asking whether or not this even belongs here or if this should go in a contrib space somewhere.

[slinkydeveloper]

Hi @kconwayinvision, thanks for the proposal. We would really love to see the protobuf format, what about starting with a PR for the implementation of format.Format?

For the datacodec implementation, let's figure it out after the format

[kconwayinvision]

A pull request for the Format is up here: #662

[kconwayinvision]

If there are folks watching this issue and not the PRs then #688 is waiting on reviews.

[kconwayinvision]

This is ready to close.

Format support added: #662
Codec support added: #688