Add support for IsDelta in return types

[mikepizzo]

OASIS has added support for an IsDelta annotation that specifies that action or function parameters or return types may be represented as a delta (that is, they may contain deleted entities or nested changes). See https://github.com/oasis-tcs/odata-specs/blob/main/docs/odata-csdl-json/odata-csdl-json.md#annotation-coreisdelta) for a description of applying the annotation to a return type.

For example, the following "getChanges" function returns a collection of users, represented as a delta payload:

  <Function Name="getChanges" IsBound="true">
    <Parameter Name="bindingParameter" Type="microsoft.graph.savePoint" />
    <ReturnType Type="Collection(microsoft.graph.user)">
      <Annotation Term="Org.OData.Core.V1.IsDelta" Bool="true" />
    </ReturnType>
  </Function>

This Issue is to add support for this annotation to OpenAPI.NET.OData.