update(userspace/sysdig): minor improvements for next release

[jasondellaluce]

This refactors the codebase by:

• Fixing a couple of bugs in plugin_utils when loading plugins from a YAML file
  • It can now also be used mixed with of -H and -I. To determine which plugin is the input one, -I has priority over the config file to allow easy overrides
• Creating two new utils sources for replacing the deprecated list_events and list_fields libsinsp's functions
• Moving source files under a utils directory and make header imports more concise and explicit
• Fixing the printed list of events with the -L option: it now contains the real event list, also containing the generic ones
• Updating -l, --list, --list-markdown to print fields for all loaded event sources (plugins included), also optionally allowing to print fields for one source selectively

[jasondellaluce]

Applied your suggestion and rebased on top of dev!