Allow running multiple importer jobs at the same time

[ctron]

As this issue already has links established to relevant sub-issues, let's re-purpose this issue to be the User Story that defines the expected functionality for the first downstream release.

As a TPA user I want to have 4 importers preconfigured; Github Security Advisory data (enabled), Vulnerability (CVE) data (enabled), Red Hat SBOM (disabled) and Red Hat Vex (disabled) and able to execute in parallel so that I can efficiently populate the application with data from external sources.

Acceptance criteria:

GHSA Importers enabled by default to load data in temporal reverse order (latest first) based on GHSA git repository folders structure
CVE Importer enabled by default to load data in temporal reverse order (latest first) based on CVE git repository folders structure
RH SBOM Importer by default disabled
RH VEX Importer by default disabled
Importers run in parallel

The corresponding downstream issue is TC-2218

The original issue description is below, and this still adds value.
A single instance of the importer service should be able to run multiple importer runs at the same time.

Also, multiple instances of the importer service should be able to distribute importer runs between them.

I think it is worth solving those two problems at the same time, since the might be related and might use a common foundation of sharing the load.