Use targetPlatform when installing plugin from open-vsx

[msujew]

What it does

Closes #11291
Closes #13558

For a while now open-vsx has offered to query plugins using the targetPlatform filter. This adjusts Theia's open-vsx integration to respect the backend's target platform. This is related to #12410, which implemented this feature for the CLI.

When using the filter, only plugin entries with the specified target platform are actually returned. That's why we have to also query universal after querying the platform specific plugins to ensure that we actually install the newest available extension.

Also closes #11291 by using the new v2 API endpoint which returns more compact results.

How to test

Testing this works best when you have two different operating systems available. I used Windows (PC) and Ubuntu (Gitpod) to test this.

1. Install a plugin with native dependencies such as the rust-analyzer
2. Assert that the plugin starts up when opening a *.rs file (it currently crashes on master if you aren't accidentally on the correct OS)
3. Assert that all the other plugin related features work as expected

Follow-ups

I'm not sure this is how the open-vsx API is intended to be used. Querying the API multiple times for different target platforms (i.e. the backend platform + universal) seems a bit counterintuitive. We can maybe improve on this.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[jfaltermeier]

Looks good to me. I've tested it on Ubuntu and in a Windows VM, and the expected extensions were installed correctly. I've added two comments/questions inline for clarification.

[jfaltermeier]

Thanks, lgtm!