[WIP] feat: add parallel-ssh (PSSH) connector implementation

[wowi42]

WORK IN PROGRESS

Add new PSSH connector with support for parallel SSH operations using the parallel-ssh library. Includes comprehensive test suite covering connection handling, command execution, and file operations.

• Pull request is based on the default branch (3.x at this time)
• Pull request includes tests for any new/updated operations/facts
• Pull request includes documentation for any new/updated operations/facts
• Tests pass (see scripts/dev-test.sh)
• Type checking & code style passes (see scripts/dev-lint.sh)

[Fizzadar]

I think this is basically just making a parallel SSH client per host right? I don't think that's a problem it just means it's a wrapper around python-ssh2 rather than using any of pssh's functionality (again, not a problem!).

Anyway - this is very cool BUT per the contributing doc:

note that new connectors will not be merged to pyinfra, instead they should be provided as a separate installable package. PR's that link to these packages in the docs very welcome.

(Also worth noting that pyinfra 4 is likely to switch to async APIs for connectors, although that's a way off yet)