Add support for Postgres multirange

[Tom Carrick]

Postgres 14 adds various multirange data types corresponding to the existing range types: ​https://www.postgresql.org/docs/14/rangetypes.html

e.g.:

select nummultirange(numrange(null, 2), numrange(4, 5), numrange(8, 9));

Gives "{(,2),[4,5),[8,9)}".

I would like to have some support for these in Django.