Make embedding generation go through inference (#606)

This PR does the following:
1) adds the ability to generate embeddings in all supported inference
providers.
2) Moves all the memory providers to use the inference API and improved
the memory tests to setup the inference stack correctly and use the
embedding models

This is a merge from #589 and #598

llama_stack/providers/tests/inference/conftest.py

@@ -18,6 +18,12 @@ def pytest_addoption(parser):
         default=None,
         help="Specify the inference model to use for testing",
     )
+    parser.addoption(
+        "--embedding-model",
+        action="store",
+        default=None,
+        help="Specify the embedding model to use for testing",
+    )
 
 
 def pytest_configure(config):