Allow truncating embeddings to specified dimensions.

[Davidyz]

A lot of embedding models support truncating the embedding output to accelerate the query. However, not all chromadb embedding function adapters (Ollama) support this. This PR adds the option for the users to set a customised embedding dimension where appropriate.

Note: If you changed the dimension for an existing collection, you'd need to drop it and re-vectorise. Otherwise, the queries won't work because the embedding dimensions must align.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.77%. Comparing base (e6d3f6b) to head (733f6cf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #265   +/-   ##
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  Coverage   99.77%   99.77%           
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  Files          24       24           
  Lines        1767     1775    +8     
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+ Hits         1763     1771    +8     
  Misses          4        4           

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