rpc: Add Coinstats index to getindexinfo

Summary:
This is a backport of core#19521 [9 & 10/17]
https://github.com/bitcoin/bitcoin/pull/19521/commits/ca01bb8d689f93e1c7669b0ba7a4994c0206dabd
https://github.com/bitcoin/bitcoin/pull/19521/commits/655d929836a71af23d2035d2e2e99ad8b8c340c3

Depends on D11600

Test Plan: ninja && test/functional/test_runner.py rpc_misc

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11604