[net/refactor] Add manual connections to ConnectionType enum

Summary:
Partial backport (3/19) of core PR19316:
https://github.com/bitcoin/bitcoin/pull/19316/commits/1521c47438537e192230486dffcec0228a53878d

Depends on D8712.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8713