[feature] add min and max at static methods

[itsmuntadhar]

First, I would like to say thanks for the great work!

We've been looking into ULID to use instead of UUID/GUID in our projects and figured we could get rid of the dedicated "CreatedAt" column/prop that we usually have.

To make querying easier, we thought such methods would be great to have in the lib so we could do something like

var anHourAgo = DateTimeOffset.UtcNow.AddHours(-1);
var idGte = Ulid.MinAt(anHourAgo);
queryable = queryable.Where(x => x.Id >= idGte);

Was also looking into writing a couple more extensions, maybe in another PR.
Basically

DateTimeOffset.UtcNow.NewUlid();
DateTimeOffset.UtcNow.MinUlid();
DateTimeOffset.UtcNow.MaxUlid();

Thanks again