adding nebius api client

[fsndzomga]

What does this PR do?

This PR introduces the NebiusClient implementation for integrating with the Nebius API. It mirrors the functionality of the OpenAIClient but uses the Nebius API's base URL and corresponding environment variable (NEBIUS_API_KEY).

Summary:

• Added NebiusClient class that inherits from ModelClient.
• Updated the client to handle Nebius-specific configurations like base URL and API key.
• Included support for both synchronous and asynchronous calls.
• Added tests for the NebiusClient, similar to those for the OpenAIClient.

Motivation:

The Nebius API integration extends AdalFlow's capabilities to support Nebius-based models, enhancing the library's versatility.

Dependencies:

• No new dependencies were introduced for this change.

Fixes #<issue_number> (if applicable)

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Before submitting

• Was this discussed/agreed via a GitHub issue? (not for typos and docs)
  ✅ Yes
• Did you read the [contributor guideline](https://adalflow.sylph.ai/contributor/index.html)?
  ✅ Yes
• Did you make sure your PR does only one thing, instead of bundling different changes together?
  ✅ Yes
• Did you make sure to update the documentation with your changes? (if necessary)
  ✅ Yes
• Did you write any new necessary tests? (not for typos and docs)
  ✅ Yes
• Did you verify new and existing tests pass locally with your changes?
  ✅ Yes
• Did you list all the breaking changes introduced by this pull request?
  No breaking changes.

[liyin2015]

@fsndzomga Thanks for adding the pr, but we should start an issue and some discussion with the community. We are not sure how this model client is wanted by users and what value it brings. Could you follow our contributor guide https://adalflow.sylph.ai/contributor/index.html