feat: add plugins installation

[Moon1706]

This PR allows you to install plugins from an official or your own server. As I can see in practise sometimes build-in forced Pulumi plugin installation doesn't work.

[simenandre]

Hello @Moon1706! Thank you for opening this PR.

Recently, #810 was opened. From what I can understand, this is the same as what you're trying to add here. Can you confirm?

[Moon1706]

@cobraz
Exactly, but my solution is more suitable)

[Moon1706]

@cobraz I'm ready. You can check a code.
PS: Comment in the other PR - #810 (comment)

[Moon1706]

@RobbieMcKinstry could you also check it? Of course, if you'll have time for that. Previous discussion was really useful and super nice for me)
PS: Comment in the other PR - #810 (comment)

[cstrempfer]

#810 was closed, because the solution here seems to be cleaner. Can this PR be completed?

[RobbieMcKinstry]

Hello! I'm afraid I'm no longer an OSS maintainer for Pulumi, so I can't push this PR forward. Best of luck, and thanks for your continued use of Pulumi!

[iwahbe]

@Moon1706 It would be helpful if you let us know what problems you are hitting. We would like to ensure that pulumi up "just works", so the best outcome for us would be to fix the problem at the source.

The next option that comes to mind is invoking pulumi install in the GitHub action, instead of adding a plugins JSON blob to the interface.