Add composite embedders and pooling for hf models

[nnethercott]

Pull Request

Related issue

Fixes #1099

What does this PR do?

• Adds a new embedder of type CompositeEmbedder, adds pooling: PoolingOpt to HuggingFaceEmbedders
• Adds a new pytest fixture enabling the experimental feature "compositeEmbedders" and a basic test to make the client can configure a composite embedder through a POST to /indexes/{index_uid}/settings/embedders

PR checklist

Please check if your PR fulfills the following requirements:

• Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
• Have you read the contributing guidelines?
• Have you made sure that the title is accurate and descriptive of the changes?

Thank you so much for contributing to Meilisearch!