Add support for IsDelta in return types

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.

The issue names no implementation files or tests. Start by locating how OpenAPI.NET.OData processes OData annotations and return types, then use the shown getChanges function as the input case; done means its IsDelta return-type annotation is represented correctly in the generated OpenAPI document.