feat: Add topic-partition-key record grouper

[jeqo]

Closes #198

This PR includes a changelog record grouper by topic partition and key.

To cope with field requirement, users can move a field to the key using SMT:

    "file.name.template": "{{topic}}-{{partition:padding=true}}/user-id={{key}}/{{start_offset:padding=true}}.jsonl",

    "transforms": "createKey,extractInt",
    "transforms.createKey.type": "org.apache.kafka.connect.transforms.ValueToKey",
    "transforms.createKey.fields": "user_id",
    "transforms.extractInt.type": "org.apache.kafka.connect.transforms.ExtractField$Key",
    "transforms.extractInt.field": "user_id"

So it is not needed for the grouper to deal with record fields.

[jjaakola-aiven]

Some nits to clean.