Merge bitcoin/bitcoin#23077: Full CJDNS support

420695c1933e2b9c6e594fcd8885f1c261e435cf contrib: recognize CJDNS seeds as such (Vasil Dimov)
f9c28330a0e77ed077f342e4669e855b3e6b20a1 net: take the first 4 random bits from CJDNS addresses in GetGroup() (Vasil Dimov)
29ff79c0a2a95abf50b78dd2be6ead2abeeaec9f net: relay CJDNS addresses even if we are not connected to CJDNS (Vasil Dimov)
d96f8d304c872b21070245c1b6aacc8b1f5da697 net: don't skip CJDNS from GetNetworkNames() (Vasil Dimov)
c2d751abbae3811adaf856b1dd1b71b33e54d315 net: take CJDNS into account in CNetAddr::GetReachabilityFrom() (Vasil Dimov)
9b43b3b257a00f777538fcc6e2550702055a1488 test: extend feature_proxy.py to test CJDNS (Vasil Dimov)
508eb258fd569cabda6fe15699f911fd627e0c56 test: remove default argument of feature_proxy.py:node_test() (Vasil Dimov)
6387f397b323b0fb4ca303fe418550f5465147c6 net: recognize CJDNS addresses as such (Vasil Dimov)
e6890fcb440245c9a24ded0b7af46267453433f1 net: don't skip CJDNS from GetNetworksInfo() (Vasil Dimov)
e9d90d3c11cee8ea70056f69afaa548cee898f40 net: introduce a new config option to enable CJDNS (Vasil Dimov)
78f456c57677e6a3a839426e211078ddf0b3e194 net: recognize CJDNS from ParseNetwork() (Vasil Dimov)
de01e312b333b65b09c8dc72f0cea6295ab8e43f net: use -proxy for connecting to the CJDNS network (Vasil Dimov)
aedd02ef2750329019d5698b14b17d67c5a563ad net: make it possible to connect to CJDNS addresses (Vasil Dimov)

Pull request description:

  CJDNS overview
  =====

  CJDNS is like a distributed, shared VPN with multiple entry points where every participant can reach any other participant. All participants use addresses from the `fc00::/8` network (reserved IPv6 range). Installation and configuration is done outside of applications, similarly to VPN (either in the host/OS or on the network router).

  Motivation
  =====

  Even without this PR it is possible to connect two Bitcoin Core nodes through CJDNS manually by using e.g. `-addnode` in environments where CJDNS is set up. However, this PR is necessary for address relay to work properly and automatic connections to be made to CJDNS peers. I.e. to make CJDNS a first class citizen network like IPv4, IPv6, Tor and I2P.

  Considerations
  =====

  An address from the `fc00::/8` network, could mean two things:
  1. Part of a local network, as defined in RFC 4193. Like `10.0.0.0/8`. Bitcoin Core could be running on a machine with such address and have peers with those (e.g. in a local network), but those addresses are not relayed to other peers because they are not globally routable on the internet.
  2. Part of the CJDNS network. This is like Tor or I2P - if we have connectivity to that network then we could reach such peers and we do relay them to other peers.

  So, Bitcoin Core needs to be able to tell which one is it when it encounters a bare `fc00::/8` address, e.g. from `-externalip=` or by looking up the machine's own addresses. Thus a new config option is introduced `-cjdnsreacable`:
  * `-cjdnsreacable=0`: it is assumed a `fc00::/8` address is a private IPv6 (1.)
  * `-cjdnsreacable=1`: it is assumed a `fc00::/8` address is a CJDNS one (2.)

  After setting up CJDNS outside of Bitcoin Core, a node operator only needs to enable this option.
  Addresses from P2P relay/gossip don't need that because they are properly tagged as IPv6 or as CJDNS.

  For testing
  =====
  ```
  [fc32:17ea:e415:c3bf:9808:149d:b5a2:c9aa]:8333
  [fc68:7026:cb27:b014:5910:e609:dcdb:22a2]:8333
  [fcb3:dc50:e1ae:7998:7dc0:7fa6:4582:8e46]:8333
  [fcc7:be49:ccd1:dc91:3125:f0da:457d:8ce]:8333
  [fcf2:d9e:3a25:4eef:8f84:251b:1b4d:c596]:8333
  ```

ACKs for top commit:
  dunxen:
    ACK 420695c
  jonatack:
    re-ACK 420695c1933e2b9c6e594fcd8885f1c261e435cf per `git range-diff 23ae793 4fbff39 420695c`
  laanwj:
    Code review ACK 420695c1933e2b9c6e594fcd8885f1c261e435cf

Tree-SHA512: 21559886271aa84671d52b120fa3fa5a50fdcf0fcb26e5b32049c56fab0d606438d19dd366a9c8ce612d3894237ae6d552ead3338b326487e3534399b88a317a

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