Expose ability to over-control complex property serialization

[xuzhg]

Assemblies affected
Which assemblies and versions are known to be affected e.g. ASP.NET Core OData 8.x

Describe the bug
Recently, We have had customers who want to omit the null complex property.

So far, we don't have a better for them to omit null complex property. As a workaround, they have to override the "CreateSelectExpandNode" virtual method to change the "SelectExpandNode" returns from this method.

This is ugly because "SelectExpandNode" is an "entity scope" object used to control all entities' serialization for the same type.

So, we'd like to have a better "overridable" method for customers to use to control the serialization of the complex.

[corranrogue9]

We need to make sure that the client can handle these responses before supporting this on the service side.

[chrisspre]

see #657

[corranrogue9]

Can we make sure to also create the other issues where needed?

[xuzhg]

created a separate issue for omit-values prefer header at server side, see details at: #672