Add MinAt and MaxAt static methods

[itsmuntadhar]

Description

This PR adds a couple of static methods that work similarly.

MinAt:
Generates the minimum possible Ulid given a DateTimeOffset

MaxAt:
Generates the maximum possible Ulid given a DateTimeOffset

Reason

We have been using Ulid and this library for a while and it's quite great with pagination. But in order to use that, we sometimes have to do var idGte = Ulid.NewUlid(createdAtGte, new byte[] { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 }) to compare it against the primary key

Solution

Theses minute methods would make the code more readable e.g.: var idGte = Ulid.MinAt(createdAtGte)

[neuecc]

Thank you.
Given that you can pass randomness, I believe you can use a workaround.
Considering its name and nature, it might be difficult to include it as a lib standard.