cli: Add "headers" and "verificationprogress" to -getinfo

Summary:
These value are useful to know the current progress of
initial sync, or of catching up.

This is a backport of Core PR17302

Test Plan: src/bitcoin-cli -getinfo

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8220