[herald] add function to get crypto price info

Summary:
T2972

Depends on D13584

Add new function to make configurable API call to coingecko for fiat price information

Include unit tests for mock responses and a new script, npm run getPrices, to test the function with a real API call

Test Plan:
npm i
npm test
npm run getPrices and expected price info is logged to console

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13586