feat: add CancelProxy type for time-delayed proxy announcements

[thecaptain789]

Summary

This PR adds a new CancelProxy proxy type that enables users to delegate the ability to reject proxy announcements without granting other permissions.

Closes #590

Motivation

When using time-delayed proxies, users need the ability to:

• Cancel pending announcements for security purposes
• Delegate this cancel ability without granting broader permissions
• Analyze transactions before they execute and reject suspicious ones

The CancelProxy type addresses this by only allowing reject_announcement calls from the proxy pallet.

Changes

1. common/src/lib.rs

   • Added CancelProxy variant to ProxyType enum
   • Added TryFrom implementation (value: 18)
   • Added From implementation (ProxyType::CancelProxy => 18)

2. runtime/src/lib.rs

   • Added InstanceFilter implementation that only allows reject_announcement calls

Usage

Users can now add a proxy with the CancelProxy type to delegate the ability to reject pending announcements:

// Add a Cancel proxy
Proxy::add_proxy(origin, delegate, ProxyType::CancelProxy, delay);

// The delegate can then reject announcements
Proxy::reject_announcement(origin, real, call_hash);

Testing

The existing mock implementations use catch-all patterns and will handle the new variant correctly.