feat: restore provider_data constructor kwarg

[cdoern]

Summary

Adds back the provider_data: Mapping[str, Any] | None kwarg on OgxClient and AsyncOgxClient that was present in the previous llama-stack-client SDK but lost during the rename to ogx-client. When provided, it is JSON-serialized and merged into default_headers under X-OGX-Provider-Data — the header the OGX server reads to thread per-request provider credentials through.

Why

Without this kwarg, every caller upgrading from llama-stack-client has to construct the header themselves on every client instantiation:

OgxClient(
    base_url=...,
    default_headers={"X-OGX-Provider-Data": json.dumps(get_provider_data())},
)

That breaks a large surface of OGX integration tests and downstream code on upgrade. Restoring the affordance keeps the migration to ogx-client source-compatible for the constructor signature.

Behavior matches the old SDK

The previous implementation in llama-stack-client v0.7.2-alpha.3 was:

custom_headers = default_headers or {}
if provider_data is not None:
    custom_headers["X-LlamaStack-Provider-Data"] = json.dumps(provider_data)

This PR mirrors that, with the only intentional difference being the header name (X-OGX-Provider-Data, matching the renamed server).

Test plan

• OgxClient(base_url=..., provider_data={"k": "v"}) accepts the kwarg
• client.default_headers["X-OGX-Provider-Data"] contains {"k": "v"} JSON-encoded
• Same for AsyncOgxClient