Add multisig-wallet contract example

[alexgduarte]

Contributes to #11

Summary

• Add an original multisig wallet smart contract example.
• Include propose, approve, execute, proposal query behavior, messages, state, and unit coverage.
• Add a schema binary target, validate threshold configuration, and remove executed proposals so approved messages cannot be replayed.

Validation

• git diff --check

Rust validation was not run in this Windows workspace because cargo and rustc are not installed on PATH.

Bounty Contact

GitHub: @alexgduarte
Email: alexduartegomescouto@gmail.com

[alexgduarte]

Hi maintainers, quick check-in on this and the related open example PRs for #11. Several simple-contract examples are still open and mergeable from my side. Are these useful as submitted, or would you prefer that I narrow, consolidate, or close some of them before further review? Happy to adjust the batch to match the direction you want for the repository.