Move NodeImpl from interfaces/node.cpp to node/interfaces.cpp

Summary:
This is a partial backport of core#20494
https://github.com/bitcoin/bitcoin/pull/20494/commits/12bd0fc9d70333c54c83ebb08c734272dbd330c2

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12746