Add azure-pipelines build and test definitions (#20840)

Add azure-pipelines build and test definitions (#20840)

This adds an azure pipeline definition with a matrix of product and test builds, using helix to run tests. The intention is that this definition will eventually be used for both our official build and CI testing.

There is one build job for each OS/platform/arch, and one test job for each OS/platform/arch/priority/R2Rflag. The test job builds tests and then submits them to helix, passing along a number of test run modes. One helix test job will be created for each OS/platform/arch/priority/R2Rflag/helixtargetqueue/testscenario.

There is a lot of work left to be done to get this up to parity with our official builds and CI, which I've tried to call out in comments.