Merge #4456

4456: Add Ollama as an embeddings provider r=dureuill a=jakobklemm

# Pull Request

## Related issue
[Related Discord Thread](https://discord.com/channels/1006923006964154428/1211977150316683305)

## What does this PR do?
- Adds Ollama as a provider of Embeddings besides HuggingFace and OpenAI under the name `ollama`
- Adds the environment variable `MEILI_OLLAMA_URL` to set the embeddings URL of an Ollama instance with a default value of `http://localhost:11434/api/embeddings` if no variable is set
- Changes some of the structs and functions in `openai.rs` to be public so that they can be shared.
- Added more error variants for Ollama specific errors
- It uses the model `nomic-embed-text` as default, but any string value is allowed, however it won't automatically check if the model actually exists or is an embedding model

Tested against Ollama version `v0.1.27` and the `nomic-embed-text` model.

## PR checklist
Please check if your PR fulfills the following requirements:
- [x] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
- [x] Have you read the contributing guidelines?
- [x] Have you made sure that the title is accurate and descriptive of the changes?

Co-authored-by: Jakob Klemm <jakob@jeykey.net>
Co-authored-by: Louis Dureuil <louis.dureuil@gmail.com>

milli/src/update/settings.rs

@@ -1178,6 +1178,13 @@ pub fn validate_embedding_settings(
                 }
             }
         }
+        EmbedderSource::Ollama => {
+            // Dimensions get inferred, only model name is required
+            check_unset(&dimensions, "dimensions", inferred_source, name)?;
+            check_set(&model, "model", inferred_source, name)?;
+            check_unset(&api_key, "apiKey", inferred_source, name)?;
+            check_unset(&revision, "revision", inferred_source, name)?;
+        }
         EmbedderSource::HuggingFace => {
             check_unset(&api_key, "apiKey", inferred_source, name)?;
             check_unset(&dimensions, "dimensions", inferred_source, name)?;