ci: add GitHub Actions workflow to build and test Soroban contract

[eleven-smg]

What this does

Adds a GitHub Actions CI workflow (.github/workflows/rust.yml) for the Soroban smart contract.

On every push and pull request it:

• Installs the Rust toolchain and the wasm32-unknown-unknown target
• Installs the Soroban CLI
• Runs the unit tests (cargo test)
• Builds the contract for wasm in release mode (cargo build --target wasm32-unknown-unknown --release)

Notes

• The workflow runs from the contracts/ directory, where the crate lives.
• Added a step to install the libdbus/libudev system libraries, which the Soroban CLI needs to build on the CI runner.
• Scope is limited to .github/workflows/ only — no contract or source files were changed.

Proof

CI passes on my fork (all steps green): a successful run is attached below.

Closes #1

[Queenode]

Thank you @eleven-smg for your contribution to Kolo.
work welldone