net: change CNetAddr::ip to have flexible size
Summary:
Before this change `CNetAddr::ip` was a fixed-size array of 16 bytes, not being able to store larger addresses (e.g. TORv3) and encoded smaller ones as 16-byte IPv6 addresses. Change its type to `prevector`, so that it can hold larger addresses and do not disguise non-IPv6 addresses as IPv6. So the IPv4 address `1.2.3.4` is now encoded as `01020304` instead of `00000000000000000000FFFF01020304`. Rename `CNetAddr::ip` to `CNetAddr::m_addr` because it is not an "IP" or "IP address" (TOR addresses are not IP addresses). In order to preserve backward compatibility with serialization (where e.g. `1.2.3.4` is serialized as `00000000000000000000FFFF01020304`) introduce `CNetAddr` dedicated legacy serialize/unserialize methods. Adjust `CSubNet` accordingly. Still use `CSubNet::netmask[]` of fixed 16 bytes, but use the first 4 for IPv4 (not the last 4). Only allow subnetting for IPv4 and IPv6.
Completes backport (2/2) of core PR19628:
https://github.com/bitcoin/bitcoin/pull/19628/commits/102867c587f5f7954232fb8ed8e85cda78bb4d32
Depends on D9175.
Test Plan:
ninja all check-all ninja bitcoin-fuzzers
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Subscribers: majcosta
Differential Revision: https://reviews.bitcoinabc.org/D9176