Expose HeadersSyncState::m_current_height in getpeerinfo()

Summary:
This is a partial backport of core#25717
https://github.com/bitcoin/bitcoin/pull/25717/commits/03712dddfbb9fe0dc7a2ead53c65106189f5c803

Depends on D15166

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15167