[chronik-doc] Fix JSON serialization of BigInt

Summary: BigInt is not json serializable, so we monkey patch the type to add this serialization. If it fits a Number, use that instead (common case) and use a string as a safe fallback if it doesn't.

Test Plan:

npm run build
npm run serve

Check the utxos are returned and displayed properly.

Reviewers: #bitcoin_abc, alitayin

Reviewed By: alitayin

Differential Revision: https://reviews.bitcoinabc.org/D18144