Add .NET 7.0 Agent

[CharliePoole]

Add an agent for running tests under .NET 7.0. This can initially just be a copy of the standard agent built for .NET 7.0. We may wish to experiment with combining some of the references to use either 5.0 or .NET standard.

[CharliePoole]

Agent is added to version 3 but not yet to the version 4 development code. Note that we do NOT yet run tests under .Net 7.0 in our CI builds, because the SDK and runtime are installed in a non-standard location.

[CharliePoole]

This issue has been resolved in version 3.16.0

The release is available on:
GitHub.
NuGet packages are also available NuGet.org and
Chocolatey Packages may be found at Chocolatey.org