Add metadata support for documents

[aellispierce]

Add metadata column to pgvector storage to allow associating additional information with stored documents. This enables tracking metadata like source documents, page numbers, or any other document-specific information.

• Add JSONB metadata column to schema
• Update add_texts to support metadata parameter
• Update update_texts to support metadata parameter
• Add metadata support to similarity search results
• Add tests for metadata functionality

BREAKING CHANGE: Schema update requires existing tables to be recreated or manually altered to add metadata column

Addresses: https://github.com/patterns-ai-core/langchainrb/issues/318

[andreibondarev]

Sorry for the late reply!

@aellispierce Wow! This is amazing! Would you mind adding an entry to the CHANGELOG?

[andreibondarev]

I'm wondering what kind of changes would be needed here: https://github.com/patterns-ai-core/langchainrb_rails

[aellispierce]

@andreibondarev I've updated the changelog ☺️