Allow Relation#sum to take an init parameters

[casperisfine]

Ref: #41669 (comment)

This is needed to not prevent full use of Enumerable#sum on Relation objects.

Since #42080, if you are summing a list of objects that aren't Numeric, you need to provide an init value, e.g. products.sum(Money.new(0), &:price).

cc @pixeltrix @alberto-mota @rafaelfranca

[casperisfine]

An issue with this is that Relation#sum is being reported as the source of the deprecation, which really isn't helpful for fixing it up.

No idea how to improve that though.

[casperisfine]

Ok, I've updated Relation#sum further to deal with the deprecation itself. This should make it easier for users to fix the deprecations.