feat: add support for group address validation in user-exists map and implement group validation logic

[Aravinda-HWK]

📌 Description

To support group emails, the socketmap service is checking the groups from the IDP and validating the existence of the group in the correct domain.

• Closes [FEATURE] Support group email accept in the socketmap service #300

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🔍 Changes Made

• Create a group.go file to check the group existence from the IDP(use the existence auth to get the bearer token)
• Update the README.md file

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✅ Checklist (Email System)

• Core services tested (SMTP, IMAP, mail storage, end-to-end delivery)
• Security & compliance verified (auth via Thunder IDP, TLS, DKIM/SPF/DMARC, spam/virus filtering)
• Configuration & deployment checked (configs generated, Docker/Compose updated)
• Reliability confirmed (error handling, logging, monitoring)
• Documentation & usage notes updated (README, deployment, API)

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🧪 Testing Instructions

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📷 Screenshots / Logs (if applicable)

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⚠️ Notes for Reviewers

Create a group in Thunder, and the group email address will be <group_name>-group@<domain_name>.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the socketmap service by integrating group email address validation with the existing user validation mechanism. It allows the service to check for the existence of group addresses, formatted as -group@, against the Thunder IDP, ensuring that group-based email routing and permissions are correctly handled. This change expands the utility of the user-exists map to cover both individual users and designated group addresses.

Highlights

• Group Address Validation: Implemented support for validating group email addresses using the Thunder IDP within the user-exists socketmap.
• Dedicated Group Logic: Introduced a new group.go file containing the ValidateGroupAddress function to specifically handle group existence checks.
• Documentation Update: Updated the README.md to reflect the new group address validation capability and format.

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[gemini-code-assist]

Code Review

This pull request successfully adds support for group address validation by introducing a new group.go file for validation logic and updating the user-exists handler to check for groups if a user is not found. The changes are well-structured and the documentation in README.md is updated accordingly. I have one suggestion to improve code maintainability in the new group.go file by reducing code duplication.

[maneeshaxyz]

LGTM.