download:plugins should respect Version

[tsmaeder]

Feature Description:

Right now we are using the uri of an extension pack (https://open-vsx.org/api/eclipse-theia/builtin-extension-pack/1.83.1/file/eclipse-theia.builtin-extension-pack-1.83.1.vsix) to specify which built-ins we want to download. However, when we download the plugins in the built-in extension pack, we download whatever the newest compatible version of that plugin at open-vsx.org is at the time of the download. This might lead to a mix of versions and thus unpredictable builds and behavior.

We should be able to better control this. Ideas are:

1. Don't use extension packs, use individual plugin uri's
2. Allow to specify a version range for the plugin versions on the download uri
3. Allow to specify a maximum API level required for the plugins in the extension pack.