community[patch]: surrealdb provide functions for MMR (Maximal Marginal Relevance) (langchain-ai#21185)

This PR contains 4 added functions:

- max_marginal_relevance_search_by_vector
- amax_marginal_relevance_search_by_vector
- max_marginal_relevance_search
- amax_marginal_relevance_search

I'm no langchain expert, but tried do inspect other vectorstore sources
like chroma, to build these functions for SurrealDB. If someone has some
changes for me, please let me know. Otherwise I would be happy, if these
changes are added to the repository, so that I can use the orignal repo
and not my local monkey patched version.

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Co-authored-by: Harrison Chase <[email protected]>
Co-authored-by: Bagatur <[email protected]>

Author: 3 people