[csharp][generichost] Implement not required nullable properties (#16810)

* init

* fixed read and write

* completed changes using latest-nrt sample

* fixed all samples

* add null check on write, change on exception

* resolved conflicts

* build samples

* added backing property for not required properties

* more not required and nullable hanlding improvements

* revert sample updates for a merge master

* revert sample updates for a merge master

* sample build is working, need to remove warnings

* fixed warnings in .net 7 with nrt

* fixed manual tests

* fixed all samples

* fix npe

* removed debugging lines

* revert changes to unused file

* removed unused lambdas

* fix a serialization bug

* make option a hidden property

* updated documentation

* improved parameter ordering

samples/client/petstore/csharp/OpenAPIClient-httpclient/.openapi-generator/FILES

@@ -77,6 +77,7 @@ docs/PolymorphicProperty.md
 docs/Quadrilateral.md
 docs/QuadrilateralInterface.md
 docs/ReadOnlyFirst.md
+docs/RequiredClass.md
 docs/Return.md
 docs/RolesReportsHash.md
 docs/RolesReportsHashRole.md
@@ -195,6 +196,7 @@ src/Org.OpenAPITools/Model/PolymorphicProperty.cs
 src/Org.OpenAPITools/Model/Quadrilateral.cs
 src/Org.OpenAPITools/Model/QuadrilateralInterface.cs
 src/Org.OpenAPITools/Model/ReadOnlyFirst.cs
+src/Org.OpenAPITools/Model/RequiredClass.cs
 src/Org.OpenAPITools/Model/Return.cs
 src/Org.OpenAPITools/Model/RolesReportsHash.cs
 src/Org.OpenAPITools/Model/RolesReportsHashRole.cs