feat(api): Add options for supporting various embedding models (#1192)

We need to support:
- asymmetric embedding models (#934)
- truncation policies (#933)
- varying dimensional output (#932) 

## Test Plan

```bash
$ cd llama_stack/providers/tests/inference
$ pytest -s -v -k fireworks test_embeddings.py \
   --inference-model nomic-ai/nomic-embed-text-v1.5 --env EMBEDDING_DIMENSION=784
$  pytest -s -v -k together test_embeddings.py \
   --inference-model togethercomputer/m2-bert-80M-8k-retrieval --env EMBEDDING_DIMENSION=784
$ pytest -s -v -k ollama test_embeddings.py \
   --inference-model all-minilm:latest --env EMBEDDING_DIMENSION=784
```

docs/_static/llama-stack-spec.yaml

@@ -3235,6 +3235,28 @@ components:
             List of contents to generate embeddings for. Each content can be a string
             or an InterleavedContentItem (and hence can be multimodal). The behavior
             depends on the model and provider. Some models may only support text.
+        text_truncation:
+          type: string
+          enum:
+            - none
+            - start
+            - end
+          description: >-
+            (Optional) Config for how to truncate text for embedding when text is
+            longer than the model's max sequence length.
+        output_dimension:
+          type: integer
+          description: >-
+            (Optional) Output dimensionality for the embeddings. Only supported by
+            Matryoshka models.
+        task_type:
+          type: string
+          enum:
+            - query
+            - document
+          description: >-
+            (Optional) How is the embedding being used? This is only supported by
+            asymmetric embedding models.
       additionalProperties: false
       required:
         - model_id