fix(SLK-129137): Add GH App migration action.

[mjshastha]

Implemented

• enable reusable workflows to authenticate with GitHub App tokens

Description

JIRA-129137

Summary of the change in this repository

Type of change

• Chore (README.md update / pipeline update / etc)
• Bug fix (non-breaking change which fixes an issue)
• Vulnerability fix (non-breaking change without code changes)
• New feature (non-breaking change which adds functionality)
• Breaking change ((major version increased) - no backward compatibility)

Testing

• Added / Ran automated tests (unit / integration / e2e)
• Manual tests - local environment
• Manual tests - dev/staging environment

Release

• Documentation update required? (Notified PM on required changes)
• Requires UI / CICD client / scanner alignment?
• Requires infrastructure alignment?
• Requires migration?
• Has Datadog monitoring?
• No new HIGH / CRITICAL vulnerabilities? (FIPS ready / Updated KNOW_VULNERABILITIES.md)

Links to related PRs / Migration scripts / Documentation

Screenshots / Video