Server Extensions and Subsystems Enhancements

The following Server Extensions and Subsystems SPI enhancements should be done:

• move the spi from "wildfly 10" to "core" module
• add Extensions to the JBossServer, enforcing each concrete server impl has its own set of extensions
• rework task that removes unsupported extensions and subsystems from configs, to take advantage of the Extensions for each server, and provide environment properties to specify which extensions and subsystems to include/exclude in migrated configs
• add Module Finder for extensions, to ensure that any missing module in target server is migrated from the source server