This commit adds a block form of `ActiveSupport::MessageEncryptors#rotate`
and `ActiveSupport::MessageVerifiers#rotate`, which supports
fine-grained per-salt rotation options.  The block will receive a salt,
and should return an appropriate options Hash.  The block may also
return `nil` to indicate that the rotation does not apply to the given
salt.  For example:

  ```ruby
  verifiers = ActiveSupport::MessageVerifiers.new { ... }
  verifiers.rotate(serializer: JSON, url_safe: true)
  verifiers.rotate do |salt|
   case salt
   when :foo
     { serializer: Marshal, url_safe: true }
   when :bar
     { serializer: Marshal, url_safe: false }
   end
  end

  # Uses `serializer: JSON, url_safe: true`.
  # Falls back to `serializer: Marshal, url_safe: true`.
  verifiers[:foo]

  # Uses `serializer: JSON, url_safe: true`.
  # Falls back to `serializer: Marshal, url_safe: false`.
  verifiers[:bar]

  # Uses `serializer: JSON, url_safe: true`.
  verifiers[:baz]
  ```

This can be particularly useful when migrating older configurations to a
unified configuration.