add embedding model by default to distribution templates (#617)

# What does this PR do?
Adds the sentence transformer provider and the `all-MiniLM-L6-v2`
embedding model to the default models to register in the run.yaml for
all providers.

## Test Plan
llama stack build --template together --image-type conda
llama stack run
~/.llama/distributions/llamastack-together/together-run.yaml

llama_stack/apis/models/models.py

@@ -21,9 +21,10 @@ class CommonModelFields(BaseModel):
     )
 
 
-class ModelType(Enum):
+@json_schema_type
+class ModelType(str, Enum):
     llm = "llm"
-    embedding_model = "embedding"
+    embedding = "embedding"
 
 
 @json_schema_type