feat(auth): implement XOAUTH2-based SMTP authentication with client email authorization

[Aravinda-HWK]

📌 Description

This PR is to update SASL service to support M2M application authentication support(client_credentials).

• Closes [TASK] Implement XOAUTH2-Based SMTP Authentication with Config-Driven Sender Authorization #276

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🔍 Changes Made

• Add new config file to email addresses for each application.
• Support to client_credentials authentication

✅ Checklist (Email System)

• Core IMAP commands tested (LOGIN, CAPABILITY, LIST, SELECT, FETCH, LOGOUT).
• Authentication is tested.
• Docker build & run validated.
• Configuration loading verified for default and custom paths.
• Persistent storage with Docker volume verified.
• Error handling and logging verified
• Documentation updated (README, config samples).

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🧪 Testing Instructions

To test the server, use the instructions in the README in the test directory.

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📷 Screenshots / Logs (if applicable)

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⚠️ Notes for Reviewers

[gemini-code-assist]

Code Review

This pull request adds sender authorization for OAuth client_credentials grants using a JSON mapping of client IDs to allowed emails. It updates the Claims struct and SASL server to enforce these restrictions. Feedback suggests passing the Claims struct by pointer to properly update the session identity and removing redundant string trimming to clean up the implementation.

[maneeshaxyz]

Lgtm.