[avalanche] Add helper function Proof::amountToScore().

Summary:
There are multiple places we want to convert an Amount of stake into a proof score, so pull that
logic into it's own helper function. Score computation logic in tested in the Avalanche unit tests,
which covers this behavior.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11179