Mechanism for both uploading and retrieving encoders, combiners and decoders

[w4nderlust]

We want to make sure that users can upload and retrieve Ludwig models (entire ECD pipelines) but also single encoders, combiners and decoders.

So we need to both allow for uploading of single es, cs and ds, as well as specifying that, from an uploaded ecd they only want to retrieve a specific piece.

For instance if I upload a model for email classification with an encoder for the meila text, and encoder for the sender email address and a date encoder, a combiner and a category decoder, another user should be able to retrieve just the encoder of the sender email address.