OSGi: add completion for properties available in 'sling:OsgiConfig' file

[deeprim]

Given a sling:OsgiConfig file has correct name, hence we can derive corresponding OSGi service class from it, provide list of all available properties upon completion for attribute name within the file

<!-- config.dev/com.service.Service.xml -->
<?xml version="1.0" encoding="UTF-8"?>

<jcr:root xmlns:sling="http://sling.apache.org/jcr/sling/1.0" xmlns:jcr="http://www.jcp.org/jcr/1.0"
          jcr:primaryType="sling:OsgiConfig"
          <caret> <!-- list of properties derived from com.service.Service should be proposed for completion -->