Ensure one single `primary` attribute

[azmeuk]

RFC7643 indicates that multi-value primary attribute should be true only once per list:

primary
A Boolean value indicating the 'primary' or preferred attribute
value for this attribute, e.g., the preferred mailing address or
the primary email address. The primary attribute value "true"
MUST appear no more than once. If not specified, the value of
"primary" SHALL be assumed to be "false".

There should be a validator for this