implemented the extension point for coverages at r13039
- introduced the base class org.geoserver.web.data.store.StoreEditPanel to be added at the store edit page to take care of the connection parameters. Workspace, store name and enabled properties keep being in control of the main store edit page
- made DataStorePanelInfo.componentClass being StoreEditPanel
- added StoreExtensionPoints to look up the GeoServerApplication for the StoreEditPanel appropriate for a given StoreInfo and provide a default StoreEditPanel if there's no one declared for a specific store type
- introduced org.geoserver.web.data.store.DefaultCoverageStoreEditPanel that provides the input field for the coverage URL, being the default for coverage types that don't contribute to the UI
implemented the extension point for coverages at r13039