Implement Gas sponsorship relay endpoint

[gabito1451]

This pr implements the gas sponsorship relay endpoint as requested. This feature allows the backend to sponsor transactions for users by co-signing them using a hot wallet, provided the transactions target the CallRegistry contract.

Key Implementation Details:
Relay Service (

RelayService
):

Transaction Validation: Implements a strict check ensuring that every operation in the transaction is a Soroban InvokeHostFunction call specifically directed at the configured CallRegistry contract ID.
Fee Sponsorship: Uses Stellar Fee-Bump Transactions to wrap the user's signed transaction, allowing the backend to pay for gas fees without the user needing native XLM.
Hot Wallet Signing: Dynamically signs the fee-bump outer transaction using the RELAY_HOT_WALLET_SECRET.
Relay Controller (

RelayController
):

Exposes POST /relay/tx which accepts a transaction XDR.
Returns the transaction hash upon successful submission to the Stellar network.
Module Integration:

Created

RelayModule
and integrated it into the main

AppModule
.
Configured a dedicated SorobanRpc.Server for relay operations.
Configuration:

Updated

.env.example
to include the RELAY_HOT_WALLET_SECRET variable.
Files Created/Modified:

relay.service.ts
(New)

relay.controller.ts
(New)

relay.module.ts
(New)

app.module.ts
(Modified)

closes #118

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