feat: New OpenAI compat embeddings API (#2314)

# What does this PR do?
Adds a new endpoint that is compatible with OpenAI for embeddings api. 
`/openai/v1/embeddings`
Added providers for OpenAI, LiteLLM and SentenceTransformer. 


## Test Plan
```
LLAMA_STACK_CONFIG=http://localhost:8321 pytest -sv tests/integration/inference/test_openai_embeddings.py --embedding-model all-MiniLM-L6-v2,text-embedding-3-small,gemini/text-embedding-004
```

llama_stack/providers/utils/inference/embedding_mixin.py

@@ -4,7 +4,9 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
+import base64
 import logging
+import struct
 from typing import TYPE_CHECKING
 
 if TYPE_CHECKING:
@@ -15,6 +17,9 @@ from llama_stack.apis.inference import (
     EmbeddingTaskType,
     InterleavedContentItem,
     ModelStore,
+    OpenAIEmbeddingData,
+    OpenAIEmbeddingsResponse,
+    OpenAIEmbeddingUsage,
     TextTruncation,
 )
 from llama_stack.providers.utils.inference.prompt_adapter import interleaved_content_as_str
@@ -43,6 +48,50 @@ class SentenceTransformerEmbeddingMixin:
         )
         return EmbeddingsResponse(embeddings=embeddings)
 
+    async def openai_embeddings(
+        self,
+        model: str,
+        input: str | list[str],
+        encoding_format: str | None = "float",
+        dimensions: int | None = None,
+        user: str | None = None,
+    ) -> OpenAIEmbeddingsResponse:
+        # Convert input to list format if it's a single string
+        input_list = [input] if isinstance(input, str) else input
+        if not input_list:
+            raise ValueError("Empty list not supported")
+
+        # Get the model and generate embeddings
+        model_obj = await self.model_store.get_model(model)
+        embedding_model = self._load_sentence_transformer_model(model_obj.provider_resource_id)
+        embeddings = embedding_model.encode(input_list, show_progress_bar=False)
+
+        # Convert embeddings to the requested format
+        data = []
+        for i, embedding in enumerate(embeddings):
+            if encoding_format == "base64":
+                # Convert float array to base64 string
+                float_bytes = struct.pack(f"{len(embedding)}f", *embedding)
+                embedding_value = base64.b64encode(float_bytes).decode("ascii")
+            else:
+                # Default to float format
+                embedding_value = embedding.tolist()
+
+            data.append(
+                OpenAIEmbeddingData(
+                    embedding=embedding_value,
+                    index=i,
+                )
+            )
+
+        # Not returning actual token usage
+        usage = OpenAIEmbeddingUsage(prompt_tokens=-1, total_tokens=-1)
+        return OpenAIEmbeddingsResponse(
+            data=data,
+            model=model_obj.provider_resource_id,
+            usage=usage,
+        )
+
     def _load_sentence_transformer_model(self, model: str) -> "SentenceTransformer":
         global EMBEDDING_MODELS