Add optional license field?

[m-mohr]

Currently, we specify it as a string of either:

1. An SPDX license identifier
2. A string of form "title " where title is the name of the license (free-form) and the URL must point to the license text (in <>)

So for example:

1. CC0-1.0
2. Copernicus License <https://copernicus.example/license.txt>