Add a new tag group for the Eco related information about the application

[MurzNN]

The KDE project started to apply the KDE Eco initiative to take care of the energy-efficient Free & Open Source Software for long-term hardware use.

They have successfully Eco-Certified the Okular app and get the Blue Angel ecolabel, and some other applications are on the go.

And to make this feature more visible, it's good to add a new tag group, related to this information, to be able to filter apps by included in the eco initiative. Additionally, this will make this initiative more visible to regular users.

So, the suggestion is to add to the AppStream Metadata XM file spec a new tag group <eco> with a structure like this:

<eco>
  <certification>
    <label>KDE Eco</label>
    <link>https://invent.kde.org/teams/eco/blue-angel-application/-/blob/master/applications/okular/de-uz-215-eng-annex-7-okular.xml</link>      
  </certification>
  <certification>
    <label>Blue Angel</label>
    <link>https://invent.kde.org/teams/eco/blue-angel-application/-/blob/master/applications/okular/de-uz-215-eng-annex-7-okular.xml</link>      
  </certification>
</eco>