Add support for Laravel-style relations

[bennothommo]

This PR allows people to define Laravel-style relations using methods that return Relation objects. So instead of:

public $belongsTo = [
     'user' => User::class,
];

You may use:

public function user(): BelongsTo
{
    return $this->belongsTo(User::class);
}

The main reason for this is to reduce friction for porting Laravel code over to Winter, and as a preference point for people who may wish to use the more declarative style of defining relations.

I've adjusted the unit tests to demonstrate this in action, and will be porting over the relation tests in the Winter system module into Storm, as they really should be in Storm anyway.

In order to maintain parity with the array-style configuration of relations in Winter, method-defined relations will need to either implement one of the following to indicate that they provide relations, and allow certain functionality to find them as required (for example, deleting related models when deleting a primary model):

• The relation method must have a return type that correlates to a Relation class (like with the example above)
• Alternatively, the method must use the Winter\Storm\Database\Attributes\Relation attribute and define the relation type, like so:

use Winter\Storm\Database\Attributes\Relation;

#[Relation('belongsTo')]
public function user()
{
    return $this->belongsTo(User::class);
}

Todo

• Fix attachment relations and tests
• Add support for model traits with new format
• Finish porting tests from Database Tester plugin in core
• Full documentation of new syntax

[LukeTowers]

Still have performance issues with the softdelete trait

[bennothommo]

@LukeTowers I've changed the soft delete methods on the relation to be available at all times, but the actual soft delete flag will only be enabled when softDeletable() is called on the relation AND the related model has the SoftDelete trait.

[LukeTowers]

@bennothommo is there a related docs PR?

[bennothommo]

@LukeTowers yep, here: wintercms/docs#176

[bennothommo]

Late change: Using the Relation attribute to define a relation method will now require the relation type to be defined as the attribute's first parameter. Only a minor API change, but critical for behind-the-scenes to prevent infinite loops.

So instead of:

#[Relation]

It would now be:

#[Relation('hasOne')]