feat: implement grantee rage quit mechanism for #39

[edehvictor]

Pull Request: feat: implement "Rage Quit" for Grantees - Issue #39

Overview

This PR implements the "Rage Quit" security mechanism in the GrantContract. This feature empowers grantees to exit a paused stream permanently if they feel the pause is unfair. It guarantees the immediate payout of accrued funds and transitions the grant to a terminal state.

Changes

• Terminal State Logic: Added GrantStatus::RageQuitted to ensure that once a grantee exits, the stream cannot be resumed by the DAO admin.
• Security Guarding: Modified resume_stream and withdraw functions to explicitly block operations on grants with a RageQuitted status.
• Immediate Settlement: Integrated 100% payout of vested (accrued) tokens using the token::Client transfer method upon execution of the quit.
• Fairness Check: Restricted the ability to Rage Quit to only those grants that are currently in a Paused state.

Acceptance Criteria Verification

• Implement rage_quit(grant_id): New public function added to the core contract.
• Accrued Funds: Grantee claims 100% of accrued funds immediately.
• Permanent Termination: The grant cannot be resumed by the admin after this action.

Related Issues

Closes #39

[edehvictor]

@JerryIdoko, please kindly review