Add Lightning Network as transport protocol for the tower

[sr-gi]

Currently, HTTP(s) is being used as transport protocol for the interaction between the user and the tower. LN should also be added so the service can be offered using either (or both) of them.

Further info: https://www.summerofbitcoin.org/project-ideas-details?recordId=rec7YMzzxpapE1d38

This is a Summer Of Bitcoin (SOB) project. If you're considering tackling this but you are not part of SOB, please leave it until SOB is over. After that, feel free to jump in.