Support property annotation without property value

[xuzhg]

Short summary (3-5 sentences) describing the issue.

Assemblies affected

Which assemblies and versions are known to be affected e.g. OData .Net lib 7.x

Reproduce steps

OData spec supports the scenario where there's a property without value but instance annotations. For example:

{
      "age@custom.instance": 123
}

ODL should support:

1. Can write property instance annotations without value.
2. Can read property instance annotations without value.