Extend Observation Intent Type

[dr-rodriguez]

STScI has been ingesting observations encapsulating images created by the Office of Public Outreach into our CAOM database. We've needed to use intent_type=science for these, but ideally we would prefer an outreach intent for them.
Partially implemented via PR opencadc/caom2tools#171 but opening an issue here for further discussion.

[dr-rodriguez]

My 2 cents is that changing this to an extensible vocabulary, as opposed to just adding a new outreach entry in the enumeration, would probably be better.
For example, we had a placeholder in our tables for intent=engineering, though we're not using it (that said, one could argue if calibration==engineering). But maybe there really aren't too many other intents that we need to consider.