Move WalletImpl from interfaces/wallet.cpp to wallet/interfaces.cpp

Summary:
This concludes backport of core#20494
https://github.com/bitcoin/bitcoin/pull/20494/commits/629a9299b2a7241a3fa7d597cb34abcbe1af9255

Depends on D12747

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12748