Create CA events registry package

Proposal

Create a go package that contains all the Gemnasium and new DS analyzer events (metrics). Reuse the same event structures. The constructors should know how to initialise each event.

Implementation plan

• Create a new project under security-analyzers
• Follow the handbook instructions around new projects
• Add events for gemnasium and ds analyzer
• Extract common events
• Add comprehensive Readme
• Add changie to be used for releases
• Advertise the project in AST
• Migrate Gemnasium
• Migrate DS analyzer