Add method to calculate input for a trampoline fee

[dpad85]

This PR adds a method in TrampolineFee that returns the exact amount that must be provided to the trampoline payment handler in order to entirely consume a given amount, without leaving any dust.

Specifically, this will allow a Phoenix wallet to send all its balance over LN.

Also, tests have been added for the existing Trampoline.calculateFees method

Example:

• My balance is 1_000_000 sat
• The trampoline fees are 4 sat + 0.4%
• The amount that must be sent to spend the balance is 992_032 msat, the fee would be 7_968 msat.