Feature request: Export finalized Liquid transaction before broadcast

### Use Case
In order to automate depositing funds at exchanges for arbitrage activities with L-BTC and L-USDT, it would be very useful to be able to store transactions in HEX format before sending, so an automated system could broadcast them later if certain conditions are met (price, balances at exchanges, etc). Like PSBT, I think PSET could be a very powerful tool for many other use cases.

### Functionatlity requested
The UI functionality needed would be just to give the user the option to store the signed transaction in HEX format instead of sending it, so the user can export or copy and paste the transaction to store it or give it to the automated system who would broadcast it later.