Add .NET 5 target and arrange nullability attributes

[zvirja]

Decided to play a bit with nullability and enhance this project in the same time 😊 Enabled nullability checks for NET 5. As a side effect also added .NET 5 framework target.

Code was adjusted to ensure it's null-valid. I did not find any subtle bug though. Some details of the implementation:

• Nullability is enabled for .NET 5 target only, as recommended by MS. For older versions we rely on .NET 5 build.
• I disabled nullability annotations for the DSL related API (a.k.a. public surface). This way our clients do not have to tediously fight to the warnings they might get due to nullability. Not sure it's would be a real problem, so we could revisit this decision later.
• member.DeclaringType was mostly null-suppressed, as I do not expect nulls there.
• obj.ToString() type annotation allows null (against common sense IMO 😖), so I coerce value to an empty string just in case.
• I added null guards to some of the entry points from DSL related API. That is to be able later remove arguments checks in our code via subsequent PRs.

I also plan to also make a subsequent PR and use the latest C# features in our code files. Also want to remove null checks in the code where we enabled nullability. If you have strong opinion against that please speak 😄

P.S. Also had to maintain the project (update SDK and Ruby on CI) to make sure I can build the project.

[dtchepak]

This looks great @zvirja , thanks!

I've left comments but approving as they are all due to me not being familiar with non-nullable ref types yet 😂

[zvirja]

@dtchepak I updated a bit of code after I found out that we could use T? for generic types. Please give it another look if you would like. Otherwise, I'll simply merge it tomorrow 😊

[dtchepak]

LGTM 👍

Thanks answering my questions 😄