Switch to Bintray/JCenter/JFrog OSS for publishing of artifacts

Due to reported constant problems with uploading to ossrh we need to see and check how can we use the repositories provided by Bintray (JCenter and JFrog OSS).

**Checklist/progress/TODO:**
- [x] Add subplugin for Knotx/knotx-gradle-plugins (`jcenter-publish`)
- [x] Verify connection with Bintray, and the ability to upload packages
- [x] Figure out if there exist a possible way of handling snapshots (as this is not available by default in the unpaid plan of Bintray for OSS projects)
- [ ] Implement and test Artifactory as a subplugin (plugin that can handle snapshots as well as releases)
- [ ] Verify that the developed solution works well in simple and composite Knot.x modules/projects

Start with the completed Knotx/knotx-gradle-plugins jcenter-publish subplugin and the remaining Artifactory subplugin task. Implement and test snapshot and release publishing, then verify the solution in simple and composite Knot.x modules or projects.

Written by the indexing model from the issue text.