Introduce ReadPasswordWithContext

[grisu48]

Introduced the ReadPasswordWithContext function that allows to read a password from the terminal with support for cancellation. This is a suggestion to address golang/go#31180.

This is a work-in-progress PR for collecting the first feedback. Please do not merge!

[grisu48]

Hey there, this PR is an attempt to resolve golang/go#31180. The current state is notready but I'd like to collect the first feedback, if this the idea is in principle worth following up and if someone has a bett
er approach to resolve the issue.

Two notes on this PR:

• The Windows version does not work currently and I'm stuck there. The used f := os.NewFile(uintptr(h), "stdin") construct is not closeable and AFAICS does Windows not support nonblocking IO access to the terminal. Any suggestion on how
  to unblock this is more than welcome
• The Unix reader works by nonblocking UI and polling, which is not ideal. If anyone has an idea for an alternative approach ...

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