Add ability to define a "top level" category when mirroring repos using tycho-p2-extras-plugin

[andrew-tram]

A feature request based off of this - https://github.com/eclipse-tycho/tycho/discussions/4422

Currently when mirroring repos, using tycho-p2-extras-plugin:mirror, if available, the org.eclipse.equinox.p2.name from the respective repositories are used and displayed at the top level.

I'd like the option to define my own "top-level" category that then expands to the repos that are being mirrored.

Example:

• My Eclipse projects --- Buildship Extra --- Build Extras - Incubating --- Buildship: Eclipse Plugins for Gradle --- etc etc etc

[andrew-tram]

Making another push for this feature.

[laeubi]

@andrew-tram do you like to propose a PR for this feature?

Beside that, if it is crucial to your business you can find dedicated support options for Tycho here: https://github.com/eclipse-tycho/tycho#getting-support

[andrew-tram]

@laeubi , sure thing. I'll look into implementing this.

[laeubi]

FYI @HannesWell who has worked on improving udpate sites recently.