Implement Gas Relay endpoint

[gabito1451]

I have implemented the Gas Relay endpoint as requested. The implementation ensures that the backend co-signs and sponsors transactions (covering gas fees) only for the specific CallRegistry contract.

Key Changes:
Refined

RelayService
:

Robust Validation: Added a strict check in

validateTransaction
to ensure that all operations in the transaction are invokeHostFunction calls specifically targeting the CallRegistry contract (retrieved via

ConfigService
).
Fee-Bump Implementation: Correctly implements Stellar Fee-Bump transactions. It wraps the user's transaction in a fee-bump envelope where the relayer's hot wallet is the fee source.
Security Checks:
Ensures the inner transaction has at least one signature (user's signature).
Verifies transaction expiration (timeBounds) to prevent the relayer from paying for stale transactions.
Added a small fee margin (500 stroops) to Ensure the fee-bump is accepted by the network even during slight fee fluctuations.
Proper Submission: Uses SorobanRpc.Server.sendTransaction and handles the response correctly, including logging the errorResultXdr if submission fails.
Controller Verification:

Confirmed the POST /relay/tx endpoint in

RelayController
correctly handles the XDR input and communicates with the

RelayService
.
Environment Integration:

The relayer uses RELAY_HOT_WALLET_SECRET for signing, which follows the project's established structure.

closes #118