Track the index of the key expression in PubkeyProvider

Summary:
Backport of Core PR18204 part [2/7] : https://github.com/bitcoin/bitcoin/pull/18204/commits/df55d44d0de2174ba74ed3a28bef5e83b0a51b47

Depends on D8381

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8382