[token-server + cashtab] Implement captchas for free XEC rewards

Summary: To combat scripted looting of Cashtab's Free XEC rewards (and support high rewards in the future), implement recaptcha v3 on the rewards.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16854