Add support to Shelley mainnet in app/Main.hs

It seems that the entrypoint file `app/Main.hs` only support Byron transaction, so it can't be used in cardano mainnet. Anyone can add the support of Shelley?

Start by reading app/Main.hs and tracing how the existing Byron transaction path is selected and constructed. Determine the Shelley mainnet requirements from the surrounding project context, then consider the work complete when the entrypoint can construct and sign Shelley mainnet transactions as well as Byron transactions.

Written by the indexing model from the issue text.