Expose SamplePtr age in C++ API

In order to sort Samples by age as was discussed as a necessity in workshop https://github.com/orgs/eclipse-score/discussions/1076 , the C++ API needs to expose the sample's age.

Start by locating the C++ API definition of SamplePtr and read the linked workshop discussion for the expected age semantics. Trace how Samples are represented and exposed, then verify that callers can access age for sorting; the issue is done when the API exposes that value.

Written by the indexing model from the issue text.