Integrate protobuf definitions for model types in apm-server

[simitt]

Follow up on #36

Phase 2

Beta Give feedback

1. Update apm-server to use protoc-generated Go types everywhere, and removing handwritten model types
2. Update apm-data to decode events directly into protobuf-generated types, and remove the old model types
3. Update apm-server to encode events to protobuf for tail-based storage

[kruskall]

For the testplan or whoever ends up testing this, here are a few things to test/look out for:

• models are no longer using map[string]any directly, instead the data from APM Agents is mapped to structpb.Struct and then converted to map[string]any in modeljson. There is an additional point of failure in this approach because the creation of structpb.Struct could fail. If that happens, we lose data.
• protobuf models are using pointers everywhere. Look out for nil pointer dereference. event.Foo.Bar was safe before but now it's not.
• IPs are stored as strings as opposed to netip.Addr. Make sure we are not converting an invalid IP (invalidNetip.String() will produce "invalid ip", we probably shouldn't encode that).
• protobuf models are passing by reference (they are not safe to copy), if we need a copy we should use proto.Clone

[kruskall]

The listed tasks have been completed. Closing this.