Implementing .NET Dapr SDK-wide nullability annotations

[WhitWaldo]

Description

This is a WIP PR, but opening it so I don't forget about it locally. In short, this implements the <nullable>enable</nullable> attribute across all .NET Dapr SDKs both on the type level and with a few changes to the logic as necessary to facilitate the changes.

I couldn't leave the challenge posted here unanswered.

Issue reference

We strive to have all PR being opened based on an issue, where the problem or feature have been discussed prior to implementation.

Please reference the issue this PR will close: #523

That said, this isn't applicable only to Dapr.Client but aims to be applicable across the entire Dapr .NET SDK. While Dapr.Client is completed and builds right now, ASP.NET Core support is nearly completed, Actors is underway and I just noticed that both Dapr.Extensions.Configuration and Dapr.Workflow already implement this, so already done there.

• Dapr.Actors (WIP)
• Dapr.Actors.AspNetCore
• Dapr.AspNetCore (WIP)
• Dapr.Client
• Dapr.Extensions.Configuration
• Dapr.Workflow
• Samples/Examples
• Quickstarts (will need to wait until this is merged as all the NuGet imports will lack the annotations)

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Extended the documentation