Support private key forwarding to winscp

[artu-ole]

Fixes #5106
Fixes #3480
Fixes #6126

Naive ssh key transfer via temporary file converting it to ppk format that is the only one supported in cli arguments in winscp. https://winscp.net/eng/docs/public_key#private
There are couple of additional dependancies in the code.
First one, tmp-promise, already used in some other modules, like tabby-electron. Used for the temporary file creation as winscp does not accept private key directly from stdin
Second one, node-forge, merely used to convert the private key into putty format and is something that would not be needed in an ideal world. Unfortunatelly, sshpk putty private keys support(TritonDataCenter/node-sshpk#76) did not come with proper putty files writer. We'd need to either make a pull request to that upstream project or to Eugene's fork of it.

And finally, even though current sftp functionality is sufficient for 95% of work, making a more sophisticated sftp client(#6319) would be the way to go in the future for a one stop shop experience.

P.S. also, ugly timeout is there not to dispose of a temporary file beforehand and because we don't get a child process reference to hook into.

[Eugeny]

Thanks! I'm on vacation until net week and will check it out then.

[artu-ole]

Sure, no rush, In the end this might be more of a proof of concept rather than final implementation due to aforementioned shortcomings.
Getting rid of timeout is fairly simple, should be good enough if exec returned the result of execFile, something along the lines:

await this.platform.exec(path, args)
tmpFile?.cleanup()

Though that would make tabby wait to remove the file until winSCP is closed(or tabby itself is closed as tmp-promise library states that it will clean files on process exit by itself)

There might be a deeper issue lurking. It does not feel entirely safe saving user's private key into a temporary directory rather than some secure application storage. Though I'm not too well versed with electron to suggest a better solution right away.

[Eugeny]

Made exec async. It's very unfortunate that WinSCP doesn't accept private keys through environment vars which would be both convenient and perfectly safe. A workaround could be to use WinSCP's agent auth by spinning up a temporary SSH agent, but this will conflict with an already running Pageant since there's no way to run multiple agents on Windows.

[Eugeny]

Tested and it LGTM - let me know if the changes look ok to you and I'll merge ✌️

[artu-ole]

Yep, LGTM, works like a charm