Add a new API endpoint to support package-first mode

[michaelehab]

As a part of the ongoing GSoC 2025 project titled "On-demand live evaluation of packages and Integration with VulnTotal", this issue should focus on setting the base for the upcoming changes. It should add a new API endpoint that takes a list of importers to run with the package-first mode. Package-first means that it will import the advisories relevant to the specific PackageURL in real time.