add: extend pgvector vector search functionality

[muzammilvd]

Description

This PR extends the functionality of the PGVectorStore class, specifically its build_query method. Previously, the build_query method only supported cosine vector similarity. However, the pgvector-python library provides support for other vector similarity methods, which were not accessible through the current implementation of PGVectorStore.

To address this, I have added support for additional vector similarity methods in the build_query method by introducing a parameter. This allows users to specify the similarity method they want to use, instead of being restricted to cosine similarity. Now, users can take advantage of all the similarity methods offered by pgvector-python.

Fixes # (issue)

New Package?

Did I fill in the tool.llamahub section in the pyproject.toml and provide a detailed README.md for my new integration or package?

• Yes
• No

Version Bump?

Did I bump the version in the pyproject.toml file of the package I am updating? (Except for the llama-index-core package)

• Yes
• No

Type of Change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Your pull-request will likely not be merged unless it is covered by some form of impactful unit testing.

• I added new unit tests to cover this change
• I believe this change is already covered by existing unit tests

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added Google Colab support for the newly added notebooks.
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I ran make format; make lint to appease the lint gods

[logan-markewich]

uhhhh this is not the proper way to do this. It should be an attribute defined on the class earlier, like all the other attributes (table_name, schema_name, etc.)

[muzammilvd]

sure let me make changes