Add sponsorship agreement to OSGi WG

From https://gitlab.eclipse.org/eclipsefdn/infrazilla/-/issues/761#note_1004654

Also, please add the Sponsorship Agreement https://www.eclipse.org/org/workinggroups/sponsorship/working-group-sponsorship-agreement.pdf to the OSGi tile on https://www.eclipse.org/org/workinggroups/explore.php.