MC: Metadata Constants

[lovromazgon]

Introduce a constant for a record metadata field opencdc.collection in opencdc.proto used to indicate the source/destination collection of a record.

Also create a new proto specifically for metadata constants (e.g. metadata/constants.proto). It should include existing constants defined in connector.proto, which would expose them not only to connectors, but also processors. We are also missing constants populated in DLQ records (currently only defined as Go constants).

All constants should be tied to Go constants and have a getter and setter method on the type opencdc.Metadata