Enable dynamic update of Azure OpenAI model deployment name

[pmalarme]

Implements dynamic updating of the Azure OpenAI model deployment name in the backend and documents the new functionality.

• Environment Variable Integration: Fetches the Azure OpenAI model deployment name dynamically from an environment variable AZURE_OPENAI_DEPLOYMENT_NAME, allowing for runtime configuration without code changes.
• New REST Endpoints: Adds /model-deployment-name with GET and POST methods to get the current model deployment name and update it dynamically, respectively. This facilitates easy management of the model deployment name without direct environment variable access.
• Documentation Update: Enhances backend/README.md with instructions on setting the AZURE_OPENAI_DEPLOYMENT_NAME environment variable and details on using the new REST endpoints for managing the model deployment name.

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For more details, open the Copilot Workspace session.