[2.1] Added support for customizing tenant model

[dmyers]

The benefit of this is that it allows applications to customize a bit of the model say to add more attributes to fillable or add relationships to other models which is what I'm looking to do since I have added a foreign key to another table in my database. It also resolves #27.

For example all I would have to do with these changes is update the config/saml2.php file with this:

return [
    'tenantModel' => \App\Models\Saml2Tenant::class,
];

And then I can subclass this package's model in my application and I'm able to start using the new one right away to customize for my use cases.

[breart]

The same here.

Btw, we could also have something like Saml2::newModelIntance() to have one resolver function

[dmyers]

Would that return the Model::newModelInstance() or Model::query() that it currently uses? Not sure which is preferred in that case.

Like I have this change stashed if you like it:

<?php

namespace Slides\Saml2;

use Slides\Saml2\Models\Tenant;

class Saml2
{
    public static function tenantModel()
    {
        $class = config('saml2.tenantModel', Tenant::class);
        return $class::query();
    }
}

[dmyers]

@brezzhnev Do you think this looks good or have any thoughts on changing it more?

[dmyers]

@brezzhnev Any thoughts here? I'd really like to be able to use this and customize the model without forking or anything if you have time to review.