Advisory versioning to avoid duplication and track changes

ATM whenever import pipeline encounters an advisory with modified content, it treats it as a new advisory. This makes it harder to track changes in advisories over time. Instead of treating modified advisories as new ones, we should start versioning the advisory. To do this we can use aliases like CVE, GHSA, PySec. Whenever pipeline receives a modified content for given alias, it should store the modified content as a new version of the existing advisory (imported from the same pipeline).