DDSPSMC11_ — Unintuitive way to access extension member functions

Invoking an extension function in a standard type requires using an overloaded arrow operator. For example:

// Standard class (in dds namespace)
dds::domain::DomainParticipant participant(MY_DOMAIN_ID);
// Call a standard method
participant.assert_liveliness();
// Call an extension method:
participant->vendor_specific_method(...);

The use of the arrow operator is counterintuitive to most people (the variable is not a pointer) and didn't allow an IDE to show the available extensions if the regular dot operator was used.

We propose adding an "extensions()" function to dds::core::Reference and dds::core::Value that can be used as follows:

// Call an extension method:
participant.extensions().register_durable_subscription(...);