OpenCDC proto

[lovromazgon]

Description

This PR adds the following:

• OpenCDC proto files moved from conduit-connector-protocol to this repo, as they will be shared between the connector and processor SDK.
• The repo now contains a GitHub action to validate proto files* and upload them to the schema registry on pushes to main.
• Added FromProto and ToProto methods on Record and related types. Methods are tested and have benchmarks. Compared to the equivalent methods in conduit-connector-protocol the methods are roughly 20% faster and will allow us to relieve the pressure on GC in the future as we will be able to reuse proto structs instead of allocating new ones each time.
• Added UnmarshalJSON method on Record which allows you to unmarshal a JSON record. Previously unmarshalling failed because opencdc.Data is an interface.

*Note that the validation action will fail in this PR, because it compares the file with the previous state in main and checks for breaking changes. The action right now doesn't find any proto files in main, so it fails.

Quick checks:

• I have followed the Code Guidelines.
• There is no other pull request for the same update/change.
• I have written unit tests.
• I have made sure that the PR is of reasonable size and can be easily reviewed.

[hariso]

LGTM!