Merge VSTS CI source into Azure Pipelines CI source; add local support for VSTS request source

[sphanley]

This PR provides cleanup and bugfix/enhancement work aimed at improving the level of support for Azure Pipelines. Changes include:

• Merging ci_source/vsts.rb into ci_source/azure_pipelines.rb, combining their functionality as appropriate. As was noted as a p.s. on Fix run with Azure Pipelines as CI Source and Azure Repos Git as Request Source #1284, Azure DevOps (which contains Azure Pipelines) is the new name for what used to be Visual Studio Team Services, so these two CI sources were providing two partial implementations for what's actually one CI platform. With this change, azure_pipelines.rb provides a unified implementation which should support running on Azure DevOps pipelines, with repositories hosted in Azure Devops or elsewhere.

• Making the changes necessary to support the VSTS request source from a local repository, via danger pr {pr_url}. By setting DANGER_VSTS_API_TOKEN and DANGER_VSTS_HOST, users can now test against an Azure DevOps pull request locally.

Worth noting is that while the VSTS request_source would likely be more appropriately renamed to "AzureGit" or something similar, I held off on making this change only because DANGER_VSTS_API_TOKEN and DANGER_VSTS_HOST reference the "VSTS" nomenclature, and I didn't want to cause a breaking change.

[orta]

Perfect, PR looks great to me, needed a few reads before I was certain on it 👍🏻

Nice work, and well explained

[orta]

Shipped as 9.2.0

[sphanley]

Thanks! Yeah, it's a little bit to take in at once, but my team really appreciates the work that y'all have put into Danger and wanted to contribute back a bit since we've been using it in the Azure DevOps environment. I appreciate the quick review!