Add analyzers for .NET Core 3.0

These are raw meeting notes that will be cleaned up later by @shirhatti

### Useful vs-threading analyzers for async usage

VS Threading has a bunch of useful analyzers that we'd like to push into the various parts of the core SDK. Here's the summary of things we decided about each of them.

https://github.com/Microsoft/vs-threading/blob/master/doc/analyzers/index.md#diagnostic-analyzers

* VSTHRD001 - not interesting without JTF
* VSTHRD002 - analyzer very interesting everywhere. Code Fix may fail, so maybe not interesting.
* VSTHRD003 - not interesting without JTF
* VSTHRD004 - not interesting without JTF
* VSTHRD010 - interesting to UI framework, has code fix
* VSTHRD011 - interesting AFTER asynclazy is in the framework.
* VSTHRD012 - not interesting without JTF
* VSTHRD100 - interesting for Core
* VSTHRD101 - interesting for Core
* VSTHRD102 - no. too nuanced
* VSTHRD103 - interesting for Core
* VSTHRD104 - interesting for Core (2nd tier)
* VSTHRD105 - Core tier 2
* VSTHRD106 - No
* VSTHRD107 - interesting to Core
* ??? - catch returning a Task from a method within a using block, without awaiting
* VSTHRD108 - Interesting for UI
* VSTHRD109 - Yes for UI stack
* VSTHRD110 - Yes for Core
* VSTHRD111 - Yes for Core
* VSTHRD200 - Yes for Core

### Analyzers for API usage

* HttpClient usage
* Don't await Task.Run in ASP.NET Core context
* Methods you should avoid in an async context
* It'll be data driven e.g. Monitor.Enter, BlockingCollection.Add*
* Block Task.Factory.StartNew and ContinueWith
* Always create TaskCompletionSource with TaskCreationOptions.RunContinuationsAsynchronously
* Never use TaskContinuationOptions.RunContinuationsAsynchronously in tcs ctor
* Always dispose CancellationTokenSource(s)
* Always flow CancellationToken(s) to APIs that take a CancellationToken
* Prefer async using vs using in and async method if the type has IAsyncDisposable
* Prefer async/await over directly returning Task (maybe)

### ASP.NET Core

* Do not cache the access to IHttpContextAccessor.HttpContext
* Do not capture services injected into the controllers on background threads