llms/mistral: Implementing embeddings.EmbedderClient for Mistral and an example with PGVector

[mathiasb]

This PR implements the embeddings.EmbedderClient interface for mistral, using the "mistral-embed" model. There is an example, heavily inspired by the openai-embeddings-example of how it can be used with pgvector.

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[mathiasb]

I did a replace in my mod file for my local dev, and was unsure how to handle that in the PR, which I think is the reason for the CI failures. Happy to take pointers on if I should change something in my repo to fix it. KR

[mathiasb]

Ok, I think good to go for reviewing. Also, as said earlier, any pointers or feedback welcome as this is my first PR in this project.

[mathiasb]

@FluffyKebab , happy to hear any further improvement suggestions or feedback.