Add optional 'license' section to EPT specification

[hobu]

The top-level of EPT needs an optional license entry added. Some other possible sub-elements might include

• shortname from https://spdx.org/licenses/
• text full text of license
• other fields as deemed necessary.

It would be nice to leverage existing standards work that describes how to communicate data licensing in this area, but there doesn't appear to be too much. Plenty of it in open source software land, but that's not quite the same thing.

[connormanning]

Perhaps this is better achieved through something like STAC pointing at EPT rather than within EPT itself.