Full Rabet wallet support

## Context
Rabet is a popular Stellar extension.

## Goal
Implement RabetAdapter.

## Contributor guide
1. **Directory:** `sdk/`
2. Research Rabet's API (rabet.getPublicKey, rabet.sign).
3. Implement and test.
4. Check network compatibility.
5. Add to supported wallets list.

## References
- Rabet documentation.