[net/refactor] Add feeler connections to ConnectionType enum

Summary:
Partial backport (4/19) of core PR19316:
https://github.com/bitcoin/bitcoin/pull/19316/commits/0e52a659a2de915fc3dce37fc8fac39be1c8b6fa

Depends on D8713.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK, majcosta

Reviewed By: #bitcoin_abc, PiRK, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8714