Use factories to provide filters/formatting

Instead of having hard-coded support for syscall/k8s_audit events, use
the notions of filter factories/formatter factories to provide generic
support for events having a given source:

- Within the engine, maps m_filter_factories / m_rulesets /
  m_format_factories map from a given source to something that can
  create filters, hold filters, and create formatters for a given
  source. The hard-coded sinsp_factory/json_factory objects are removed.

- The specific add_xxx_filter/process_xxx_event are general purpose
  and take an event source.

- A new method create_formatter() takes a source/output format and
  provides a shared_ptr to a formatter than can resolve format
  strings. This is used by the falco outputs code.

- In falco main, create the syscall/k8s_audit filter and formatter
  factories and pass them to the engine. Later, we might make this
  configurable/selective.

With all of the above changes, the falco engine doesn't need a direct
inspector any longer, so remove it.

Signed-off-by: Mark Stemm <mark.stemm@gmail.com>

userspace/engine/falco_common.cpp

@@ -48,11 +48,6 @@ falco_common::~falco_common()
 	}
 }
 
-void falco_common::set_inspector(sinsp *inspector)
-{
-	m_inspector = inspector;
-}
-
 void falco_common::init(const char *lua_main_filename, const char *alternate_lua_dir)
 {
 	ifstream is;