SDK: implement write/transaction flows

[Omoboi-dev]

`HorizonClient` only reads. Add methods that build, simulate, sign (via wallet callback), and submit: `registerPolicy`, `payPremium`, `depositLiquidity`, and the monitor's `reportDrainEvent`.

Acceptance criteria

• Each method returns a built/submitted transaction result.
• Signing is injected (caller supplies a `signTransaction(xdr)` fn) to stay UI-agnostic.
• A short usage example in the SDK README.