VCIO-next: Migrate Redhat importers to new Advisory models

[pombredanne]

This require extensive refactoring to support advisories.

The goal is to move to use their CSAF inputs and make RHSA first-class citizen.

Getting these is made easier because at https://security.access.redhat.com/data/csaf/v2/advisories/ :

• we can get an index
• we can get all CSAF docs at once

Also:

• We are dropping OVAL that is no longer maintained at https://access.redhat.com/security/data

We are not yet considering the likely redundant CSAF VEX, SPDX SBOM and OSV data tracked separately:

• Import RedHat CSAF VEX https://access.redhat.com/security/data #1955
• Import RedHat SPDX SBOM https://security.access.redhat.com/data/sbom/beta/spdx/ #1956
• Import RedHat OSV data https://security.access.redhat.com/data/osv/all.json #1957