Add cross-contract communication contract

[Trovic1]

• Added contracts/cross_contract/src/lib.rs:1 with a new Soroban cross-contract messaging contract: message queue, route registry, optional
  callbacks, rate limiting, audit trail, and atomic/non-atomic delivery modes.
• Added contracts/cross_contract/src/test.rs:1 with coverage for routing, callback execution, callback override, queue behavior, rate limiting,
  error propagation, and atomic rollback expectations.
• Registered the crate in Cargo.toml:3 and added contracts/cross_contract/Cargo.toml:1.

closes #260

[drips-wave]

@Trovic1 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits