Implement a database abstraction layer that will enable the use of different DB backends.

[Davidyz]

Part of #221.

This will most likely be incompatible with the existing configuration, in the sense that we'd need to follow similar patterns for embedding functions and rerankers. As a temporary solution, we could maybe add a function that transforms the old config to the new one internally.

I'm not committed to this implementation, but I need some hands-on experience to know what we'd need from the abstraction layer. If this works out, we could just go with this.
Having spent some time looking into langchain implementations, I thought their approach is a bit bloated for our simple RAG tool that specialises in local files that are organised in directories (and makes extensive use of metadata). As such, I decided to follow this PR and implement my own database connector (mostly based on chromadb API design), which we can then use to implement supports for new databases.

[codecov]

Codecov Report

❌ Patch coverage is 99.80139% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.76%. Comparing base (a8b5e0a) to head (c59136b).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/vectorcode/database/chroma.py	99.09%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #282      +/-   ##
==========================================
+ Coverage   99.72%   99.76%   +0.03%     
==========================================
  Files          25       32       +7     
  Lines        1838     2091     +253     
==========================================
+ Hits         1833     2086     +253     
  Misses          5        5              

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[Davidyz]

For the sake of easily configuring database settings for all projects, I'm planning to modify the config file resolution so that project configs will be merged with the global config. This means you can only configure the db/embedding/reranker only once in the global config.

[Davidyz]

As a proof-of-concept, I'll try to get chromadb 1.x working as part of this PR. This is likely going to introduce packaging change. Specifically, the default chromadb version constraint will be <2.0.0, with an optional dep group that pin to ==0.6.3.