feat: multi-method support and automatic container pool cleanup

[eswar-7116]

Overview

This PR expands the invocation capabilities of Glambdar and improves resource management by ensuring that container pools are properly cleaned up when functions are removed.

Key Changes

• Multi-Method Support: The /invoke/:name endpoint now supports GET, POST, PUT, PATCH, and DELETE requests. The HTTP method is correctly captured and passed through to the function worker via the req.method field.
• Automatic Pool Cleanup: Implemented a DeletePool method in the PoolManager to immediately drain idle container channels and stop all associated Docker containers when a function is deleted.
• Robustness & Verification:
  • Updated the standard valid.zip test function to handle null headers and echo request metadata.
  • Added comprehensive integration tests verifying all supported HTTP methods.
• Documentation: Updated CHANGELOG.md for the v2.2.1 release.

Verification

• Integration Tests: Ran TestInvoke_Methods and TestInvoke_HappyPath with RUN_INTEGRATION_TESTS=1. All 6 method variants passed successfully.
• Manual Verification: Confirmed that warm containers are forcefully removed from Docker upon function deletion.

Commits

• ab56760 - fix: clear container pool on function delete
• 7b5b5c7 - feat: add support to other HTTP methods
• c06a952 - docs: update CHANGELOG.md