Gate execute_proposal to proposer or allow-listed executors

[ameeribro4-sudo]

Summary

Implemented a production-ready fix while maintaining the existing architecture and coding standards.

Changes

• Added NotInExecutorAllowlist error variant to GovernanceError
• Added ExecutorAllowlist storage key for managing the executor allow-list
• Added is_admin_config_key helper to identify admin-protected config keys (admin, required_approvals, executor_allowlist)
• Added set_executor_allowlist (admin-only) and get_executor_allowlist public functions
• Updated execute_proposal to check the allow-list for admin-key proposals, while still allowing the proposer to execute their own proposals
• Regular (non-admin) config keys remain executable by anyone after sufficient votes
• Wrote 6 new tests: empty allow-list, admin set, non-admin rejected, blocked non-allow-listed caller, allowed proposer, allowed executor
• All 20 tests pass
• No warnings
• Ensured no unrelated changes were introduced

Closes #123

[snowrugar-beep]

LGTM