Introduce protobuf definitions for model types

[axw]

For several reasons we would like to define an efficient, stable, binary encoding for model types. e.g. we would use this for storing events in Badger for tail-based sampling. These would be much faster to encode/decode, and more importantly will have strong stability guarantees

To achieve the above, we will define our intermediate, in-memory/on-disk, model types in protobuf -- this will be the source of truth. We'll take a phased approach to this, given that the existing types are used all across apm-server, and making a big-bang change would carry a significant amount of risk of introducing bugs.

With #35 merged, we have created a cleaner separation between the model types and the way they are encoded to JSON. The model types no longer have to directly map to the final document structure, though for our sanity we should probably keep them close. The model types do not need to be ECS-compliant, and we can instead evolve the JSON encoding over time without changing the model types.

Phase 1 (iteration-05)

• introduce protobuf definitions (probably worth basing off @marclop's work in https://github.com/elastic/apm-ingest-queueless)
• generate Go types from protobuf (into model/modelpb or something like that)
• introduce code for JSON encoding protobuf types by transforming to internal/modeljson types, like we're doing with model.APMEvent now
• introduce code for mapping model events to the protobuf-generated types; remove the code for translating from model types to modeljson, and instead translate model types to protoc-generated types, then protoc-generated types to modeljson

Phase 2 (iteration-06)
#52

[simitt]

moved phase 2 to a dedicated issue, @kruskall please close this when everything from phase-1 is done

[kruskall]

The last PR was merged today so this can be closed 🎉