Add job matrix generation scripts

[benbp]

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.

These changes are live for the .NET tests, so this PR is just a port of the code that already exists in the azure-sdk-for-net repository. The intent is to standardize it across the language repos, and then migrate all the common code into azure-sdk-tools. I will make a follow-on PR with the actual configuration changes to use this functionality in the pipelines, because I want to keep the changes separate to make reviewing/reverting easier.

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