Bugfix: PGVector/RAG - Calculate the Vector Size based on Model Dimensions

[Knucklessg1]

Why are these changes needed?

Dynamically calculate the vector column in the pgvector db based off the sentence embedding model's dimension size.

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[thinkall]

Thanks @Knucklessg1 , could you also add an example of embedding_function for using customized embedding functions in the docstrings?