Refactor + cpluginv2

[lovromazgon]

Description

This PR moves package cpluginv1 to cplugin/v1 and deprecates it. Instead, it introduces cplugin/v2. The functionality stays the same, however:

• v2 is using config/v1/parameter for the parameter types
• v2 adds support for batching records in the source and destination stream

The protocol provides packages cplugin/client (which was previously placed in the Conduit repo) and cplugin/server. Both ensure backwards compatibility for connectors, so new connectors should work with older Conduit instances and old connectors should work with newer Conduit instances.

Having the package cplugin/client in the protocol opens a way for us to write other tools that can run connector plugins (for instance a standalone acceptance test tool or connector validation utility).

I also added tests for the non-trivial from/to proto transformations. The tests make it clear we need to add schema support, as we lose types in structured data during the transformation.

Related to ConduitIO/conduit#1433.

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]

Great work, Lovro! 🚀 Left a couple of non-blocker comment only.:)

[hariso]

💥