community[patch]: DuckDB VS - expose similarity, improve performance of from_texts (#20971)

3 fixes of DuckDB vector store:
- unify defaults in constructor and from_texts (users no longer have to
specify `vector_key`).
- include search similarity into output metadata (fixes #20969)
- significantly improve performance of `from_documents`

Dependencies: added Pandas to speed up `from_documents`.
I was thinking about CSV and JSON options, but I expect trouble loading
JSON values this way and also CSV and JSON options require storing data
to disk.
Anyway, the poetry file for langchain-community already contains a
dependency on Pandas.

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>
Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>
Co-authored-by: ccurme <chester.curme@gmail.com>

docs/docs/integrations/vectorstores/duckdb.ipynb

@@ -14,7 +14,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "! pip install duckdb langchain-community"
+    "! pip install duckdb langchain langchain-community langchain-openai"
    ]
   },
   {
@@ -86,7 +86,7 @@
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "Python 3",
+   "display_name": "Python 3 (ipykernel)",
    "language": "python",
    "name": "python3"
   },
@@ -100,9 +100,9 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.12.2"
+   "version": "3.9.1"
   }
  },
  "nbformat": 4,
- "nbformat_minor": 2
+ "nbformat_minor": 4
 }