Public Maven dependency for direct ingest

We can split out a module that contains only the dependencies needed for direct ingest, eg. ingest-api. That'll be used as a client library for getting data into Sleeper.

We should ensure that minimal code is included as this will be a public API. Publishing this will establish a public contract and any clients may couple to any part of it.

We could look into flagging internal packages which will not be exposed by default.