Fix false positive in UniquenessValidator with conditions

[etiennebarrie]

First, a validation can be defined with options specific to the validator:

class Article < ApplicationRecord
  validates :user, uniqueness: { if: -> { false } }
  # equivalent to
  # validates :user, uniqueness: true, if: -> { false }
end

And this should not add an offense, like when the :if is on the validates, because we don't know what the condition is and it could lead to a false positive.

Secondly, there's a :conditions option that can't be added on validates directly because it's specific to the UniquenessValidator, which can also lead to false positives, because it can refer to columns or otherwise reduce the uniqueness in a way that prevents a unique index from covering it in the schema.

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[koic]

Thanks!