feature: Supports Entra/AD auth for Azure OpenAI

[markwaddle]

This PR adds support for Azure Entra (formerly Azure AD) authentication for the Codex CLI when connecting to Azure OpenAI services.

Changes

• Added support for Azure Entra authentication via @azure/identity dependency, allowing users to authenticate using Azure credentials without requiring an explicit API key
• Added support for configuring Azure OpenAI deployment names, allowing for cases where deployment names differ from model names
• Updated documentation in README to clarify Azure authentication options
• Improved proxy handling for both OpenAI and Azure connections
• Removed prompt for OpenAI API key on startup when another provider is specified by the user
• Refactored agent-loop.ts to use shared OpenAI client creation logic in openai-client.ts
• Made AZURE_OPENAI_API_KEY optional when using Azure provider (falls back to Entra auth)

Dependencies

• Added @azure/identity version ^4.10.0 to package.json

These changes enable users who work with Azure OpenAI services in enterprise environments where Azure Entra authentication is required.

Fixes issue #1056

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[markwaddle]

I have read the CLA Document and I hereby sign the CLA

[payneio]

Nice!

[bkrabach]

This is great! A big help for those of us with policy that prevents use of API keys but want to leverage Codex CLI at work.