enhancement(socket sink): support unix datagram mode

[jpovixwm]

Summary

Same as #21762 but with different user-facing configuration.
Adds unix_datagram as a valid value for the mode option of the socket sink.
Not supported on macOS.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

How did you test this PR?

make test on x86_64 Ubuntu 22.04 running within WSL, and:

cargo test --lib --no-default-features --features=sinks-socket --target=x86_64-pc-windows-gnu

sources:
  dummy_logs:
    type: "demo_logs"
    format: "syslog"
    count: 3

sinks:
  emit_console:
    inputs:
      - "dummy_logs"
    type: "console"
    encoding:
      codec: "text"
  emit_syslog:
    inputs:
      - "dummy_logs"
    type: "socket"
    mode: "unix_datagram"
    path: "/tmp/test_socket"
    encoding:
      codec: "text"
    framing:
      method: "newline_delimited"

Listener:
nc -lkuU /tmp/test_socket (from package netcat-openbsd)

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the "no-changelog" label to this PR.

Checklist

• Please read our Vector contributor resources.
• If this PR introduces changes Vector dependencies (modifies Cargo.lock), please
  run dd-rust-license-tool write to regenerate the license inventory and commit the changes (if any). More details here.

References

• Closes: Add support for UNIX datagram mode to socket sink #5269 (but it's already closed)
• Undoes: chore(socket sink): Revert addition of unix datagram mode #22113
• Original PR: enhancement(socket sink): support unix datagram mode #21762

[pront]

@jpovixwm thank you for re-opening this PR!

I will take a look at the implementation details but I will leave the UX aspects to @jszwedko 🙏