Add extension for Intent (therapy, prophylaxis)

[costateixeira]

Options are:

1. Extension
2. Slicing Category
3. Slicing Reason

Option 2 and 3 have the issue that we'd most likely need a discriminator based on a ValueSet which is an example binding, so the extension would not be directly reusable by implementers.

Meeting 11-03: we agree on extension.

[rlindstrm]

eHN Patient Summary and Hospital Discharge Report guidelines have intendedUse in addition to reason/indication. Good candidate for requesting an official extension or addition to base spec.