fix: model register missing model-type and not accepting metadata

[Bobbins228]

What does this PR do?

[Provide a short summary of what this PR does and why. Link to relevant issues if applicable.]
Added the ability to specify the model type when registering models.
Also fixed a bug with passing metadata which would result in the following error:

Error Type: BadRequestError                                                                                                                                     │
│ Details: Error code: 400 - {'error': {'detail': {'errors': [{'loc': ['body', 'metadata'], 'msg': 'Input should be a valid dictionary', 'type': 'dict_type'}]}}}

Closes: #215

Test Plan

Run the following commands

# Note a Llama Stack Server must be running
# Create a venv
uv sync --python 3.12
# Install the LSC with the new code changes
uv pip install -e .
# List the available models
llama-stack-client models list
# Register the granite-embedding-30m embedding model NOTE must have sentence-transformers as an inference provider
llama-stack-client models register granite-embedding-30m --provider-id "sentence-transformers" --provider-model-id ibm-granite/granite-embedding-30m-english --metadata '{"embedding_dimension": 384}' --model-type embedding
# Verify the embedding model added are present
llama-stack-client models list

[varshaprasad96]

The changes look good. @Bobbins228 just one nit - could you please add the output and use of this command to the docs (in detailed tutorial - https://llama-stack.readthedocs.io/en/latest/getting_started/detailed_tutorial.html#step-3-run-client-cli), so that users are aware that there is an easy way to register the model even after server has started. Thanks!

/lgtm
/approve

[Bobbins228]

@varshaprasad96 Happy to update docs as a follow on when this is merged 👍

[ashwinb]

Could you add this PR to the https://github.com/llamastack/llama-stack-client-python repo instead? The meta-llama repo is now read-only. We will archive it shortly.

[raghotham]

add the allowed values for Model type in the help.

[raghotham]

small nit. thanks for the change.

[Bobbins228]

Thanks @ashwinb and @raghotham I'll provide it there and close this after

[Bobbins228]

Closing in favour of #11