Add job matrix generation scripts

[benbp]

This PR is a port of functionality that is currently duplicated across the net/java/python repositories. The intent was to settle on an implementation before moving it to the /eng/common/scripts directory. After merge to this location, I'll update the net/java/python and js repos to point to the eng/common/scripts location and remove the scripts from eng/scripts.

Here is the PR text used against the other repos for reference:

This adds scripts, docs and samples supporting dynamic, cross-product matrix generation for azure pipeline jobs.
It aims to replicate the cross-product matrix functionality in github actions,
but also adds some additional features like sparse matrix generation, cross-product includes and excludes, parameter grouping and matrix filters.

This functionality is made possible by the ability for the azure pipelines yaml to take a dynamic variable as an input
for a job matrix definition (see the code sample at the bottom of the linked section).

See the README.md file for more details on the config file syntax and usage, as well as implementation details.

The tests (test-matrix-functions.tests.ps1) contain a lot of detail on expected data structures at various processing stages. The -test-matrix-functions.ps1` file could perhaps be split up or use some more organization, so let me know if it's hard to navigate.

Example:

{
  "displayNames": {
    "true": "TestFromSource"
  },
  "matrix": {
    "Agent": {
      "ubuntu-18.04": { "OSVmImage": "MMSUbuntu18.04", "Pool": "azsdk-pool-mms-ubuntu-1804-general" },
      "windows-2019": { "OSVmImage": "MMS2019", "Pool": "azsdk-pool-mms-win-2019-general" },
      "macOS-10.15": { "OSVmImage": "macOS-10.15", "Pool": "Azure Pipelines" }
    },
    "JavaTestVersion": [ "1.8", "1.11" ],
    "AZURE_TEST_HTTP_CLIENTS": [ "okhttp", "netty" ]
  },
  "include": [
    {
      "Agent": {
          "ubuntu-18.04": { "OSVmImage": "MMSUbuntu18.04", "Pool": "azsdk-pool-mms-ubuntu-1804-general" }
      },
      "JavaTestVersion": "1.11",
      "AZURE_TEST_HTTP_CLIENTS": "netty",
      "TestFromSource": true
    }
  ]
}

Sparse matrix job generation in a pipeline: https://dev.azure.com/azure-sdk/internal/_build/results?buildId=705622&view=results

Related discussion: microsoft/azure-pipelines-yaml#20

[azure-sdk]

The following pipelines have been queued for testing:
java - template
java - template - tests
js - template
net - template
net - template - tests
python - template
python - template - tests
You can sign off on the approval gate to test the release stage of each pipeline.
See eng/common workflow

[benbp]

Sample pipeline configs passing: https://dev.azure.com/azure-sdk/playground/_build/results?buildId=742512&view=results

[azure-sdk]

The following pipelines have been queued for testing:
java - template
java - template - tests
js - template
net - template
net - template - tests
python - template
python - template - tests
You can sign off on the approval gate to test the release stage of each pipeline.
See eng/common workflow

[azure-sdk]

The following pipelines have been queued for testing:
java - template
java - template - tests
js - template
net - template
net - template - tests
python - template
python - template - tests
You can sign off on the approval gate to test the release stage of each pipeline.
See eng/common workflow

[azure-sdk]

The following pipelines have been queued for testing:
java - template
java - template - tests
js - template
net - template
net - template - tests
python - template
python - template - tests
You can sign off on the approval gate to test the release stage of each pipeline.
See eng/common workflow

[azure-sdk]

The following pipelines have been queued for testing:
java - template
java - template - tests
js - template
net - template
net - template - tests
python - template
python - template - tests
You can sign off on the approval gate to test the release stage of each pipeline.
See eng/common workflow

[ghost]

Hello @azure-sdk!

Because this pull request has the auto-merge label, I will be glad to assist with helping to merge this pull request once all check-in policies pass.

p.s. you can customize the way I help with merging this pull request, such as holding this pull request until a specific person approves. Simply @mention me (@msftbot) and give me an instruction to get started! Learn more here.

[benbp]

/check-enforcer evaluate