Add nullable reference type support to schema builder

[dstelljes]

This change enables the schema builder to analyze nullable reference type annotations when configured with the NullableReferenceTypeBehavior.Annotated option. Per #102, it also makes Annotated the default behavior as it matches the current default behavior (None) for oblivious types and transparently enables better schema generation for non-oblivious types.

.NET 6 introduces new reflection APIs to support reading metadata. Since dropping support for everything below .NET 6 isn’t an option and reimplementing the parsing logic would be non-trivial, I opted to copy over the implementations to the base library’s internal infrastructure namespace. This is basically what every shim library (e.g. Nullability.Source) does as well.

The --nullable-references flag on dotnet avro create is changed from a boolean to an enum to support the annotated option, which becomes the default. The --nullable-references flag on dotnet avro generate is new, and for now it’s off by default.