Add JsonProperty(DefaultValueHandling.Include) for required primitives

[michaelcfanning]

We do not emit JsonProperty(DefaultValueHandling.Include) on properties that are marked required. If we do not do this, then the default handling for non-attributed properties is subject to being overriden by settings provided to the JSON.NET serialization classes.

When JsonProperty is applied, this value can't be overridden. This is important in some scenarios, such as SARIF, where eliminating a default value (like a boolean primitive, e.g. Run.ExecutionSuccessful) will invalidate the file.

This bug turned up in the SARIF SDK, where a log writer incorrectly initialized the serializer with DefaultValueHandling.Ignore.