Adding analyzer to notify user classes that implement IGrain should inherit from Grain

[RiskRunner0]

Closing #6194

From the feedback, it might be helpful to have an analyzer check to see if a non-abstract class implements IGrain, but doesn't inherit from Grain.

Previously, there was no message from the compiler when a user forgets to derive an implementation from the base Grain class, so this analyzer will generate ORLEANS0003 and inform the user that "Non-abstract classes that implement IGrain should derive from the base class Orleans.Grain".

Example, using HelloWorld from the Samples, where HelloGrain implements IHello, which implements the interface IGrainWithIntegerKey:

Another addition is the CodeFixProvider, which provides the user with a potential fix:

Also works on classes that derive from an abstract class that implements IGrain

[RiskRunner0]

I'd like some opinions on this, the CodeFixProvider isn't necessary but I think it's a nice to have, but it feels like cheating to use Orleans.Grain instead of adding a using statement and having just Grain.

[ReubenBond]

Looks great, thanks Matt! Could you add tests to the Analyzers.Tests project?

[RiskRunner0]

Oh, sure didn't see that project, thanks.

[ReubenBond]

Thanks :) We should add a test for abstract classes, but we can do that later.