Check attributes syntax

[Éloi Rivard]

RFC7644 §3.10 indicates that attributes have a defined syntax.
There should be a validator that checks the attributes syntax.
The validator should optionally read a resources parameter in the validation context, so it can check if the attributes urn related to a known model.
The validator should optionally read a default_resource parameter in the validation context, so it can recognize attributes without a schema urn prefix.