Merge #19628: net: change CNetAddr::ip to have flexible size

102867c587 net: change CNetAddr::ip to have flexible size (Vasil Dimov)
1ea57ad674 net: don't accept non-left-contiguous netmasks (Vasil Dimov)

Pull request description:

  (chopped off from #19031 to ease review)

  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). Do not accept
  invalid netmasks that have 0-bits followed by 1-bits and only allow
  subnetting for IPv4 and IPv6.

  Co-authored-by: Carl Dong <contact@carldong.me>

ACKs for top commit:
  sipa:
    utACK 102867c587
  MarcoFalke:
    Concept ACK 102867c587
  ryanofsky:
    Code review ACK 102867c587. Just many suggested updates since last review. Thanks for following up on everything!
  jonatack:
    re-ACK 102867c587 diff review, code review, build/tests/running bitcoind with ipv4/ipv6/onion peers
  kallewoof:
    ACK 102867c587

Tree-SHA512: d60bf716cecf8d3e8146d2f90f897ebe956befb16f711a24cfe680024c5afc758fb9e4a0a22066b42f7630d52cf916318bedbcbc069ae07092d5250a11e8f762

src/util/strencodings.cpp

@@ -318,6 +318,18 @@ bool ParseInt64(const std::string& str, int64_t *out)
         n <= std::numeric_limits<int64_t>::max();
 }
 
+bool ParseUInt8(const std::string& str, uint8_t *out)
+{
+    uint32_t u32;
+    if (!ParseUInt32(str, &u32) || u32 > std::numeric_limits<uint8_t>::max()) {
+        return false;
+    }
+    if (out != nullptr) {
+        *out = static_cast<uint8_t>(u32);
+    }
+    return true;
+}
+
 bool ParseUInt32(const std::string& str, uint32_t *out)
 {
     if (!ParsePrechecks(str))