[lbussell]

This PR adds a minimal Azure DevOps HTTP client library. The purpose is to make adding Azure DevOps support to the GitAutomation library as simple as possible.

I chose to implement a new HTTP client library instead of using Microsoft.TeamFoundationServer.Client because that library has a huge dependency tree, when we would only need to use a very small part of it. Additionally, the Azure DevOps API is extremely stable.

Part of:

• Use GitHub App Authentication for UpdateDependencies tool dotnet-docker#6345
• Remove dependency on Microsoft.DotNet.VersionTools #1658

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[lbussell]

I removed the AzureDevOps prefix from the model types, because 1) some names were getting unreasonably long, and 2) the GitAutomation.AzureDevOps namespace prevents conflicts downstream.

After double-checking the rest of the API models, I also found that committer email address can be null if 1) the commit doesn't have an email address, or 2) if the http requests are unauthenticated. I updated the model accordingly.