[faucet] Convert amounts from bigint to number for json serialization

Summary: The amount is guaranteed to fit into a Number and the json serialization don't know how to serialize a BigInt by default.

Test Plan: Trivial

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18768