Implementing .NET Dapr SDK-wide nullability annotations

[WhitWaldo]

This is just an issue to track the larger initiative to implement nullability annotations across the whole of the .NET SDK. It's a large enough undertaking that it's worth splitting into individual PRs for each SDK rather than trying to handle across all the SDKs in a single swoop.

Target APIs:

• Dapr.Actors
• Dapr.Actors.AspNetCore
• Dapr.AspNetCore
• Dapr.Client - implemented by Implementing nullability annotations on Dapr.Client SDK #1317 , pending acceptance
• Dapr.Extensions.Configuration
• Dapr.Workflow
• Dapr.Jobs - following acceptance of PR Added .NET client for Dapr Jobs API #1331
• Samples/Examples
• Quickstarts - will likely need to wait until all this is completed as the NuGet imports will lack the annotations

[WhitWaldo]

/assign

[WhitWaldo]

Following up with a note here that this issue will not be completed in the short term. Rather, it'll be part of a larger effort to migrate out all the existing functionality to standalone packages by purpose. Those packages themselves will be nullability annotated from the start, conform to best practices and start from a place of rigorous unit and integration testing as part of the transition.