Support pgvector retriever integration

[shamspias]

Add full support for the pgvector retriever in the configuration module. This includes the following updates:

• Implement the make_pgvector_retriever function to connect to a pgvector index.
• Update make_retriever to support the pgvector option.
• Add support for async mode in the PGVector retriever initialization.
• Improve type annotations and error handling to enhance stability.
• Update the README and example.env files with relevant documentation and environment variable examples for pgvector configuration.

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Impact:

This feature will improve the flexibility and ease of using pgvector within the project, providing better support for connecting to pgvector indexes.