Start of policy support

[Frassle]

This is not complete, but it's enough to get the first two conformance tests we wrote for policy passing.

This is pretty much a copy of the Go SDK (which heavily copied from the python and nodejs SDKs), plus taking some ideas from the C# provider SDK.

A few things worth calling out.

1. I've added a nuget cache clear to the start of the language tests. Me and Brandon both wasted loads of time when dealing with dotnet conformance tests because nuget was picking up old builds of the core SDK from caches. No more, we know nuke the cache before running the tests.
2. I had to add WaitDelay to the run plugin code. Seemed the dotnet run of the analyzer plugin could leave stdio pipes open even though it had terminated. Without WaitDelay run plugin would block forever on cmd.Wait().
3. Some parts of the policy SDK are partly filled in (Like config schema and properties) but not working and not currently tested. I think they're about the right shape, but we might tweak them once we have conformance tests in place to check them.

[rosieks]

ReportViolation is redundant. There's no reason to write method name in summary.