gui: Replace interface::Node references with pointers

Summary:

No change in behavior. Replacing references with pointers allows Node
interface creation to be delayed until later during gui startup next
commit to support implementing -ipcconnect option

Partial backport (3/4) of bitcoin-core/gui PR35:
https://github.com/bitcoin-core/gui/pull/35/commits/102abff9eb6c267af64f2a3560712147d1896e13

Depends on D8654.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8655