Merge bitcoin/bitcoin#27375: net: support unix domain sockets for -proxy and -onion

567cec9a05e1261e955535f734826b12341684b6 doc: add release notes and help text for unix sockets (Matthew Zipkin)
bfe51928911daf484ae07deb52a7ff0bcb2526ae test: cover UNIX sockets in feature_proxy.py (Matthew Zipkin)
c65c0d01630b44fa71321ea7ad68d5f9fbb7aefb init: allow UNIX socket path for -proxy and -onion (Matthew Zipkin)
c3bd43142eba77dcf1acd4984e437759f65e237a gui: accomodate unix socket Proxy in updateDefaultProxyNets() (Matthew Zipkin)
a88bf9dedd1d8c1db0a9c8b663dab3e3c2f0f030 i2p: construct Session with Proxy instead of CService (Matthew Zipkin)
d9318a37ec09fe0b002815a7e48710e530620ae2 net: split ConnectToSocket() from ConnectDirectly() for unix sockets (Matthew Zipkin)
ac2ecf3182fb5ad9bcd41540b19382376114d6ee proxy: rename randomize_credentials to m_randomize_credentials (Matthew Zipkin)
a89c3f59dc44eaf4f59912c1accfc0ce5d61933a netbase: extend Proxy class to wrap UNIX socket as well as TCP (Matthew Zipkin)
3a7d6548effa6cd9a4a5413b690c2fd85da4ef65 net: move CreateSock() calls from ConnectNode() to netbase methods (Matthew Zipkin)
74f568cb6fd5c74b7b9bf0ce69876430746a53b1 netbase: allow CreateSock() to create UNIX sockets if supported (Matthew Zipkin)
bae86c8d318d06818aa75a9ebe3db864197f0bc6 netbase: refactor CreateSock() to accept sa_family_t (Matthew Zipkin)
adb3a3e51de205cc69b1a58647c65c04fa6c6362 configure: test for unix domain sockets (Matthew Zipkin)

Pull request description:

  Closes https://github.com/bitcoin/bitcoin/issues/27252

  UNIX domain sockets are a mechanism for inter-process communication that are faster than local TCP ports (because there is no need for TCP overhead) and potentially more secure because access is managed by the filesystem instead of serving an open port on the system.

  There has been work on [unix domain sockets before](https://github.com/bitcoin/bitcoin/pull/9979) but for now I just wanted to start on this single use-case which is enabling unix sockets from the client side, specifically connecting to a local Tor proxy (Tor can listen on unix sockets and even enforces strict curent-user-only access permission before binding) configured by `-onion=` or `-proxy=`

  I copied the prefix `unix:` usage from Tor. With this patch built locally you can test with your own filesystem path (example):

  `tor --SocksPort unix:/Users/matthewzipkin/torsocket/x`

  `bitcoind -proxy=unix:/Users/matthewzipkin/torsocket/x`

  Prep work for this feature includes:
  - Moving where and how we create `sockaddr` and `Sock` to accommodate `AF_UNIX` without disturbing `CService`
  - Expanding `Proxy` class to represent either a `CService` or a UNIX socket (by its file path)

  Future work:
  - Enable UNIX sockets for ZMQ (https://github.com/bitcoin/bitcoin/pull/27679)
  - Enable UNIX sockets for I2P SAM proxy (some code is included in this PR but not tested or exposed to user options yet)
  - Enable UNIX sockets on windows where supported
  - Update Network Proxies dialog in GUI to support UNIX sockets

ACKs for top commit:
  Sjors:
    re-ACK 567cec9a05e1261e955535f734826b12341684b6
  tdb3:
    re ACK for 567cec9a05e1261e955535f734826b12341684b6.
  achow101:
    ACK 567cec9a05e1261e955535f734826b12341684b6
  vasild:
    ACK 567cec9a05e1261e955535f734826b12341684b6

Tree-SHA512: de81860e56d5de83217a18df4c35297732b4ad491e293a0153d2d02a0bde1d022700a1131279b187ef219651487537354b9d06d10fde56225500c7e257df92c1

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