Add support for prs backend

[Tim Visée]

This requests support for prs as backend. Please see the motivation below the four questions.

Is your feature request related to a problem? Please describe.
QtPass supports alternative backend, such as pass. prs is currently not supported.

Describe the solution you'd like
I'd like to see support for prs as alternative backend, next to pass. prs supports the passwordstore.org password store format, but is a different implementation.

Describe alternatives you've considered
I've considered changing the prs interface to be compatible with pass. This is not an option though because this breaks one of the selling points of prs.

I've considered adding a compatibility layer on top of prs, which would translate pass commands to prs. This can be very tricky though, and likely costs a lot of work.

I'd rather see prs support in QtPass itself, as Qtpass already has support for backend selection.

Additional context
This is where I'd expect prs selection to appear. Using it as pass implementation currently doesn't work.

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Motivation

prs is a secure, fast and convenient pass implementation that is cross-platform with built-in support for git sync, TOTP tokens, Tomb and other things (features).

The main goals of prs are to be faster, more convenient and more comfortable for interactive terminal use, to support many things out of the box as listed above, and to be cross-platform. I therefore think this would be a very solid choice as backend for QtPass, whereas other pass implementations I've seen are quite limited or clunky.

prs is currently not compatible with QtPass. It chooses to use a slightly different CLI API in order to remove ambiguity, its CLI is therefore not directly compatible. Because of that various errors pop up when prs is chosen as QtPass backend..

I'd love to see support for prs as alternative backend, much like how pass can be used right now. prs can be a very solid choice to achieve cross-platform support. Is this something that can be considered?

I'd be very happy to work on this in a PR but would like to ask about your opinion first.

The idea for integration was originally suggested by timvisee/prs#15.