Rename BIP32PubkeyProvider.m_extkey to m_root_extkey

Summary:
Renaming clarifies that m_extkey is actually the root
extkey that keys are derived from.

Backport of Core PR18204 part [3/7] : https://github.com/bitcoin/bitcoin/pull/18204/commits/66c2cadc91d26074b89e5ada68350b5c8676efac

Depends on D8382

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8383