net: make CNode::m_inbound_onion public, initialize explicitly

Summary:

net: make CNode::m_inbound_onion public, drop getter, update tests

net: remove CNode::m_inbound_onion defaults for explicitness

and to allow the compiler to warn if uninitialized in the ctor
or omitted in the caller.

This is a backport of core#21167

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10973