Support automatic retrival of JustJ for building with a given VM

[laeubi]

We have https://www.eclipse.org/justj/ that offers some pre-build java-packages and we already distribute it as part of the Eclipse-Packages.

What cam into my mind is, why not have something in Tycho that enables to automatically download a JustJ as an alternative for that cases where a suitable JVM is not available as either build or toolchains... this will completely remove the need to install standard JVMs and configure them, so if we cache it in the local repo it should not harm much regarding download time.

[mickaelistria]

I'm not sure it's a Tycho-specific problem; it may apply to any Java project, so maybe there is already a solution/issue report about it for Maven in general that Tycho could rely on?

[laeubi]

I have talked with the maven ppl but there where some concerns that this actually could work, so I think we can maybe prototype this here and maybe convince them later on if it gets attention from more developers...

[laeubi]

Another possible source would be https://api.adoptium.net/q/swagger-ui/

[laeubi]

I have played around with the update-site of JustJ and think what would actually be quite convenient if for a product packaging one can enable to include a JustJ so one has not to configure anything.