Allow time range defintion for applicable sources

[santilland]

There are probably multiple handlers to consider, but in general we want to be able to not only support a single datetime, but also allow for a date range (i.e. start and end time). STAC already allows this.

Concretely i can think of raw sources that only allow a datetime specification in the time property. Either we introduce new start and end time properties or we allow to specify ranges with a / in the timestring, e.g.:
"2007-03-01T13:00:00Z/2008-05-11T15:30:00Z"

Maybe other sources might also benefit from this.

See also eodash/eodash#258

[lubojr]

I'd go for option 1, start_time and end_time because we can define a date schema for that, that JSON Editor could use.

[santilland]

We could potentially use one field and use the range mode of the datetimepicker, that would solve potential issues of swapping start and end and similar things:
https://flatpickr.js.org/examples/#range-calendar