Add new reusable workflow for matrix testing scenarios

[JamesDawson]

Currently focussed on supporting .NET Framework testing scenarios.

The testPhaseMatrixJson workflow input allows you specify combinations of OSes and .NET Framework versions that tests will be run on, using the following JSON object:

{
  "os": ["ubuntu-latest", "windows-latest"],
  "dotnetFramework": ["net8.0", "net481"],
  "exclude": [
    {
      "os": "ubuntu-latest",
      "dotnetFramework": "net481"
    }
  ]
}

TODO

Update scripted-build-pipeline.yml with relevant changes from the new matrix build:

• [x] Add ability to customise runner OS
• [x] Consider removing the publishing of the Cobertura coverage XML artefact
• [x] Remove use of irongut/CodeCoverageSummary as this is now part of scripted build
• [x] Apply updates to allow running EnricoMi/publish-unit-test-result-action on different OSes
• [ ] Test workflow
• [ ] Add PowerShell step for checking runner OS to matrix build workflow