Allow custom text key to WeaviateRM - resolve issue #359

[CShorten]

Lets you specify which text key to return from a call to dspy.Retrieve

This does not incur a breaking change thanks to the Optional typing and default argument value.

This will resolve https://github.com/stanfordnlp/dspy/issues/359.

[insop]

With a cursory check, it looks great!

Hi @CShorten,

You could take this suggestion to a separate PR. I think it'd be great if you could add WeaviateRM description to this markdown docs/retrieval_models_client.md. This is a PR that adds description for other RM to this document: https://github.com/stanfordnlp/dspy/pull/400.

[okhat]

Thank you @CShorten and @insop ! Merged