TEX11 — Update section 10.5 "DDS PSM" to take advantage of new ISO IDL4 features

The DDS PSM was defined before IDL 4 was introduced and therefore could not take advantage of features that are now part of IDL 4 but were not in IDL 3.x.
IDL4.2 is now well stablished and ISO specification as well . And moreover both OpenSource and Commercial DDS vendors support it. Therefore it makes sense to update the "DDS PSM" to use these features. This will result in more natural language mappings as well as more efficient serialization.

The recommended changes are:
In 10.5 DDS PSM perform the following edits:

Replace the bullet:

• Optional attributes are mapped to a union type with a single member present when the exists case attribute is true.
  With:
• Optional attributes are mapped to IDL attributes of the same type annotated with the "@optional" annotation.

Replace the bullet:

• Specialization / Generalization PIM relationships are mapped to IDL unions. Additional data classes are introduced for generalization classes that have attributes.
  With:
• Specialization / Generalization PIM relationships are mapped to IDL structure inheritance. All the structures that participate in a Specialization / Generalization relationship are annotated with the "@appendable" annotation.

Use Optional and Appendable Annotations

The TEX specification should take advantage of the X-Types specification for the IDL/DDS PSM. This will provide additional flexibility in the future as well as more succinct and standardized definitions.
Like the use of the key annotation with respect to topic types compatibility with older DDS implementations should be maintained. This can be done with pre-processor primitives.
Optional attributes (and relations) in the PIM (cardinality 0..1) should be mapped to the @optional annotation in the PSM.
Specialisations are currently mapped into a variants union. This union should be extensible, as should the enumeration that is the switch type.