Update schema to accept more forms of software identifiers.

[alilleybrinker]

With the proposal now split into three Pull Requests, I am opening this issue to track the submissions. They are:

• Make "cpeApplicability" structure generic. #391
• Add support for Package URLs #397
• Add support for OmniBOR Artifact IDs #396

The PRs for Package URLs and OmniBOR Artifact IDs rely on #391 being merged first.

We're also working on a written "Request for Discussion"-style document outlining the motivation, background, other options, drawbacks, and more for this proposal. When it's prepared I'll update the issue to include it.