Move the Plugin Registry to its own repository

[skabashnyuk]

Description

• Create a new github repository eclipse/che-plugin-registry
• Move the Plugin Registry files from eclipse/che to eclipse/che-plugin-registry
• Add the meta.yaml needed for the 2 plugins of User Story 1 and User Story 2 of ws.next walking skeleton #10123 (c.f. [Walking Skeleton] Implement/Package plugin che-hello-world #10649)
• Add the meta.yaml needed for the theia plugin (c.f. [Walking Skeleton] Package Theia as a Che Plugin #10648)

[skabashnyuk]

done in https://github.com/eclipse/che-plugin-registry