MOVEONLY: Move NodeContext struct to node/context.h

Summary:
This is a partial backport of Core PR16839 : https://github.com/bitcoin/bitcoin/pull/16839/commits/4d5448c76b71c9d91399c31b043237091be2e5e7

Depends on D6108

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6109