[FEATURE] EDTF - support intervals

[AronNovak]

Overview of feature request
Let's extend the coverage of EDTF, it supports intervals:
https://www.loc.gov/standards/datetime/edtf.html

‘2004-02-01/2005’ is a time interval beginning sometime on February 1, 2004 and ending sometime in 2005. The start endpoint has calendar day precision and the end endpoint has calendar year precision. Similar to the previous example, the precision of the time interval at large is undefined.

But the constraint does not allow to create this.

What kind of user is the feature intended for?

Content author.

What inspired the request?

Our requirements at a client project where intervals are needed, for instance a piece of content can be created during years in some rare cases.

What existing behavior do you want changed?
The validation in EDFUtils: https://github.com/Islandora/controlled_access_terms/blob/2.x/src/EDTFUtils.php#L132

Any brand new behavior do you want to add to Islandora?

Nothing brand new, just better coverage of EDTF specs.