Assembly refactor and split

[jviau]

As a first step towards enabling devs to author re-usable libraries for DurableTask .NET sdk, we will split the assemblies up. This will allow libraries to take in a minimal amount of dependencies. Each assembly will correspond to a NuGet package of the same name. The rough assembly layout will be:

Name	Description
Microsoft.DurableTask.Abstractions	Contains base types for authoring and consuming orchestrations and activities
Microsoft.DurableTask.Generators	Contains the source generators. note: will be bundled into abstractions nuget package.
Microsoft.DurableTask.Worker	Contains the core framework for running a DTFx worker
Microsoft.DurableTask.Worker.Grpc	Contains the gRPC specific worker implementation
Microsoft.DurableTask.Client	Contains the durable task client and extensions.
Microsoft.DurableTask.Client.Grpc	Contains the gRPC client implementation for interacting with the DTFx backend

Tasks

• Split assemblies
• Public shim factory in worker package
• Common durable task builder --> enable services.AddDurableTask(b => b.UseGrpc()); secario
• Allow for re-enterable builder --> enable multiple calls to services.AddDurableTask(b => ..), all adding to the same builder.
• Adopt Mediator pattern to provide type-enforced invocations of orchestration and activities #73
• Update code gen
• Update samples.

[jviau]

Microsoft.DurableTask.Generators

We can actually pack this analyzer up with Microsoft.DurableTask.Abstractions directly, so no need to reference them separately (will still be a separate csproj). @cgillum what do you think of that?

[cgillum]

@jviau I'm not sure if it's common practice to pack analyzers together with libraries. I feel like it could be problematic since the generator package contains a bunch of private assets, though I'll admit this is beyond my knowledge of how nuget packages work.

[jviau]

That is fair. I just noticed that Microsoft.Extensons.Logging.Abstractions packages its generator along with itself.

[cgillum]

Ah, then I guess there is precedent for this! I'm fine either way. If it's not too much work, it probably makes sense to keep these tightly bound to each other.

[jviau]

Okay, I will work on that then. Won't be part of the initial PR.

[jviau]

Mediator pattern is tracked in its own issue.