Optional include of mandatory elements to the list of provided elements during serialization

[marcovisserFurore]

Description

Adding an extra setting IncludeMandatoryToElements to SerializeSetting . This is used during serialization with a set of elements:

var json = new FhirJsonSerializer(new SerializerSettings() { IncludeMandatoryToElements = true })
                .SerializeToDocument(obs, elements: new[] { "issued" });

In above case, not only the element issued is serialized but also the mandatory elements. For Observation is that status. So 2 elements are serialized here.

Related issues

Resolves #1653.

Testing

Unit test SerializationTests.IncludeMandatoryToElementsTest

FirelyTeam Checklist

• Update the title of the PR to be succinct and less than 50 characters
• Mark the PR with the label breaking change when this PR introduces breaking changes