Fix the calculation of Boltz fees

[ok300]

When calculating the Boltz service fees:

• it's always the LN invoice amount that's used as input
• the service fee is rounded up to the nearest full sat (ceil instead of round, which rounds up or down)

This is the case for both submarine and reverse submarine swaps.

This can be checked by manually entering values in https://boltz.exchange and observing the calculated Boltz fee:

• reverse submarine swap (LN -> BTC)
  • test validity of ceil
  • enter invoice amount as 50000
  • the Boltz fee is 250
    • change invoice amount to 50001
  • the Boltz fee is now 251 (round would have resulted in 250, ceil leads to the correct 251)
  • test validity of invoice amount as service fee input
  • enter receive (BTC) amount as 38356
  • Boltz fee will become 251
  • change receive (BTC) amount to 38357
  • Boltz service fee remains 251

Similar tests can be done with the reverse direction (normal submarine swaps).

Therefore, this PR:

• changes the service fee rounding to use ceil
• renames the input arg of the service fee method to explicitly state it is the invoice amount
  • "input" or "output" is somewhat vague, as "user input" can mean both "swap input" or "swap output", depending on the app's use case.

[i5hi]

Hey @ok300 Thanks for the PR! Logic and code looks good.

It fails the linting tests. I'll take care of it for now. For successive PRs make sure you run rust fmt ;)