feat: Added Nullability markup for some classes

[HavenDV]

Related to #1202

I think before that, #1202 should be split into two parts - marking the current code as is without changing the behavior
And after that, for the next big update, REQUIRED things should be replaced so that they are not null even in an empty document

This PR does not change behavior in places where null guards were provided or null was expected.
In places where there were NREs, ? were added. or other trivial fixes to avoid NRE
I'll continue with nullable markup if the change is accepted quickly enough

[darrelmiller]

We reviewed these changes and I think we understand the incremental step you are trying to take. This is consistent with the direction that we want to go for the v2 release. Thank you for making these changes. Be aware that there are likely going to still be a fair amount of code churn in this branch, so the kind of changes that still need to be made may cause merge conflicts.

[HavenDV]

It seems we need to update System.Text.Json to 8.0.4 because

/home/runner/work/OpenAPI.NET/OpenAPI.NET/src/Microsoft.OpenApi/Microsoft.OpenApi.csproj : error NU1903: Warning As Error: Package 'System.Text.Json' 7.0.2 has a known high severity vulnerability, https://github.com/advisories/GHSA-hh2w-p6rv-4g7w [/home/runner/work/OpenAPI.NET/OpenAPI.NET/Microsoft.OpenApi.sln]
/home/runner/work/OpenAPI.NET/OpenAPI.NET/src/Microsoft.OpenApi.Readers/Microsoft.OpenApi.Readers.csproj : error NU1903: Warning As Error: Package 'System.Text.Json' 7.0.2 has a known high severity vulnerability, https://github.com/advisories/GHSA-hh2w-p6rv-4g7w [/home/runner/work/OpenAPI.NET/OpenAPI.NET/Microsoft.OpenApi.sln]

It doesn't allow to pass CICD. After the update, some of the tests fail, so this needs to be done in a separate PR with update/fix for tests
Or I can add NoWarn for this temporarily.

[baywet]

@HavenDV good call.
Can you add the suppression in a similar way as this PR please? microsoft/kiota-dotnet#292

And make sure we're using version ranges for library projects.

Thanks a lot!

[baywet]

Quote reply
Reference in ne

Actually this has already been handled in #1729

[HavenDV]

Yes, I saw this so I decided not to risk making parallel changes here.
I'll have some free time today/weekend, I might take this further.

[HavenDV]

@baywet I'm not sure, but maybe it's blocked by "baywet requested changes" status. Is there anything else that needs fixing?

[baywet]

Sorry, I forgot to reset my review

[darrelmiller]

@MaggieKimani1 Could we get this PR merged before we get further conflicts?