[FEAT] Multisig support when using WalletConnect

[mihaic195]

• Add signAndSubmitTransaction method to the WalletConnect module, enabling wallets to sign and submit transactions to the network in a single step.
• This is particularly useful for multisig accounts, where the wallet can coordinate collecting all required signatures before submitting.
• The new stellar_signAndSubmitXDR method is requested as an optional WalletConnect capability, so wallets that don't support it won't be affected during session pairing.
• Add the method as an optional member of ModuleInterface so non-WalletConnect modules are unaffected.

I tested this successfully on Stellar mainnet using Lobstr and Lobstr Vault multisig when connected using WalletConnect. Original flows work the same as before, with no regressions.