[net/refactor] Introduce an enum to distinguish type of connection

Summary:

- extract inbound & outbound types

Partial backport (2/19) of core PR19316:
https://github.com/bitcoin/bitcoin/pull/19316/commits/26304b4100201754fb32440bec3e3b78cd3f0e6d

Depends on D8711.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8712