Refactor CI security scanners in a reusable composite action

[NiccoloFei]

The security scans performed as part of bake_targets.yml are quite generic and they should be refactored as reusable composite action, in order to allow reusing them as part of several other projects, such as:

• https://github.com/cloudnative-pg/pgbouncer-containers
• https://github.com/cloudnative-pg/postgres-extensions-containers
  and so on...