Proposal for a libs plugin system

Description of changes to falcosecurity/libs and /falco to support
plugins to provide events and extract fields from events.

Signed-off-by: Loris Degioanni <[email protected]>
Co-authored-by: Leonardo Di Donato <[email protected]>
Co-authored-by: Mark Stemm <[email protected]>
Co-authored-by: Leonardo Grasso <[email protected]>