Support variable length embeddings

[adri1wald]

Describe the feature or improvement you're requesting

From the blog on new embedding models:

Both of our new embedding models were trained with a technique that allows developers to trade-off performance and cost of using embeddings. Specifically, developers can shorten embeddings (i.e. remove some numbers from the end of the sequence) without the embedding losing its concept-representing properties by passing in the dimensions API parameter. For example, on the MTEB benchmark, a text-embedding-3-large embedding can be shortened to a size of 256 while still outperforming an unshortened text-embedding-ada-002 embedding with a size of 1536.

Based on the API reference, this is available via a new parameter on the embedding request called dimensions (optional integer).

Additional context

I'm happy to write a PR for this.

[dongri]

@adri1wald PR please

[dongri]

https://github.com/dongri/openai-api-rs/pull/74/files
Added dimensions

[dongri]

#74