resthub-validation: add the capacity to define custom messages per property and type

[bmeurant]

resthub-validation allows to define custom messages for constraint types per model or globally.

e.g. for a given model setting custom Min message :

...
messages: {
    'validation.Min.message': 'should be greater than {value} or equals'
}
...

this PR adds the capacity to customize a message for a constraint type AND a property :

e.g. for a given model AND property 'size' setting custom Min message :

...
messages: {
    'validation.size.Min.message': ' size should be greater than {value} or equals'
}
...

The first customization option is obviously still allowed.