Support checking provider-specific /models endpoints for available models based on key (#7538)

* test(test_utils.py): initial test for valid models

Addresses https://github.com/BerriAI/litellm/issues/7525

* fix: test

* feat(fireworks_ai/transformation.py): support retrieving valid models from fireworks ai endpoint

* refactor(fireworks_ai/): support checking model info on `/v1/models` route

* docs(set_keys.md): update docs to clarify check llm provider api usage

* fix(watsonx/common_utils.py): support 'WATSONX_ZENAPIKEY' for iam auth

* fix(watsonx): read in watsonx token from env var

* fix: fix linting errors

* fix(utils.py): fix provider config check

* style: cleanup unused imports

litellm/llms/watsonx/common_utils.py

@@ -175,7 +175,12 @@ class IBMWatsonXMixin:
 
         if "Authorization" in headers:
             return {**default_headers, **headers}
-        token = cast(Optional[str], optional_params.get("token"))
+        token = cast(
+            Optional[str],
+            optional_params.get("token")
+            or get_secret_str("WATSONX_ZENAPIKEY")
+            or get_secret_str("WATSONX_TOKEN"),
+        )
         if token:
             headers["Authorization"] = f"Bearer {token}"
         else:
@@ -245,6 +250,7 @@ class IBMWatsonXMixin:
         )
 
         token: Optional[str] = None
+
         if wx_credentials is not None:
             api_base = wx_credentials.get("url", api_base)
             api_key = wx_credentials.get(