Add benchmark to write JSON into a string

Summary:
The benchmark BlockToJsonVerbose only tests generating (and destroying)
the JSON data structure, but serializing into a string is also a
performance critical aspect of the RPC calls.

Also, use ankerl::nanobench::doNotOptimizeAway to make sure the compiler
can't optimize the result of the calls away.

This is a backport of core#21170
Depends on D13039

Test Plan: ninja bench-bitcoin

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13040