databricks: add vector search and embeddings (#25648)
### Summary
Add `DatabricksVectorSearch` and `DatabricksEmbeddings` classes to the
`langchain-databricks` partner packages. Core functionality is
unchanged, but the vector search class is largely refactored for
readability and maintainability.
This PR does not add integration tests yet. This will be added once the
Databricks test workspace is ready.
Tagging @efriis as POC
### Tracker
[✅] Create a package and imgrate ChatDatabricks
[✍️] Migrate DatabricksVectorSearch, DatabricksEmbeddings, and their
docs
~[ ] Migrate UCFunctionToolkit and its doc~
[ ] Add provider document and update README.md
[ ] Add integration tests and set up secrets (after moved to an external
package)
[ ] Add deprecation note to the community implementations.
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Signed-off-by: B-Step62 <yuki.watanabe@databricks.com>
Co-authored-by: Erick Friis <erick@langchain.dev>
