Pipeline V3

[scbedd]

This PR:

• Updates PullRequests to utilize git to run CI only packages that were changed
  • Packages that have been changed may optionally include other packages for sparse validation. For instance azure-core configures an additional 15 or so packages that must be brought in for sparse validation. This is configured solely in get_package_properties.py.
• Affected packages are now batched, so even if a PR touches 50+ packages, we will simply expand the test matrix by limiting each test run to a max of 8 packages (arbitrary limit). We just...add more test runs. Analyze will still have an issue with this many packages, I will likely multiplex that in follow-up PRs as well.
  • In the above case, when additional validation packages are pulled in due to a large number of directly affected packages, we will also sparsely include those validation packages into the test matrix. ("sparsely" translating to "on a single platform and python combination, we don't really care which")

[weshaggard]

We will want to update these triggers before merging. Given we plan to name the pipeline "python - pullrequest" I would also suggest renaming this to pullrequest.yml.

[weshaggard]

I left a few comments/questions to look at before merging and a few others that I tagged with follow-up that can come at a later time.