Add embedding task

[markstur]

Adds an embedding-retrieval task to get embedding vector for a sentence.

• Return a list of floats. For one string (a.k.a. sentence).
• Support for multiple strings removed for future batch support.
• Uses sentence-transformers.
• The data model allows for different float types (py, np.float32, np.float64).

Data model is based on Gabe's feedback in https://github.com/caikit/caikit-nlp/pull/39

This embedding service will be extended in separate PRs. With multi-task support. This service can also support sentence-similarity and reranking.

[markstur]

Tag @gabe-l-hart and will tag Anjali if I can find her.

[markstur]

rebased to catch up to main (and so I can stack PRs with multi-task support)

[markstur]

@gkumbhat anything I need to do to unblock this one? Not sure if it helps if click the "resolve conversation" buttons. Up to you.

[markstur]

@gkumbhat Updated based on feedback. I don't want to use some of the saver() code until it is fixed to be safe and even revert partial changes, but we did get some unnecessary stuff out of save(). Thanks!