Add support of Github PR URLs to a git repo URL Factory

[garagatyi]

Description

Che supports the creation of factories for a Github repo https://www.eclipse.org/che/docs/factories-getting-started.html#invoke-factory-for-a-specific-git-repo
We can add a new feature: support of PR URLs to this Factory capability.
For a link like https://che-url/factory?url=https://github.com/eclipse/che/pull/10740 we would apply a factory that clones git repo and checks out to a PR branch.
Note that there might be 2 kinds of repos for a PR:

• a branch that exists in the repo pointed by a PR URL : Move WS.NEXT flow to k8s infra implementation, rework it to use plugin broker #10740
• a branch that exists in a fork of the repo pointed by a PR URL https://github.com/garagatyi/che

We can implement one of the cases for the beginning and create a separate issue for another one, or we can implement both in the same contribution.

Reproduction Steps

OS and version:

Diagnostics:

[benoitf]

PR provided handling both cases.
just a notice: it works only for Opened PR, it doesn't work on closed/merged PR (as branch may be gone)

[benoitf]

already merged