[12.x] Add pendingAttributes method to Eloquent relationships

[AndrewMast]

Why

The withAttributes method (documented here) is very helpful for adding attributes to both the where clauses as well as the attributes when a model is made from the relationship.

However, I ran into a situation where I needed to set an attribute when the relationship is created, but not constrain the relationship to only rows where the attribute is set to that value.

The $pendingAttributes property already exists on the Eloquent builder and all the logic to add the pending attributes to the new models also exist.

What

This led to me creating the pendingAttributes method to do the same thing as withAttributes but not touch the where clauses.

Implementation:

/**
 * Specify attributes that should be added to any new models created by this builder.
 *
 * @param  \Illuminate\Contracts\Database\Query\Expression|array|string  $attributes
 * @param  mixed  $value
 * @return $this
 */
public function pendingAttributes(Expression|array|string $attributes, $value = null)
{
    if (! is_array($attributes)) {
        $attributes = [$attributes => $value];
    }

    $this->pendingAttributes = array_merge($this->pendingAttributes, $attributes);

    return $this;
}

Tests:

• tests/Database/DatabaseEloquentBelongsToManyPendingAttributesTest.php
  Copied from DatabaseEloquentBelongsToManyWithAttributesTest.php and made sure only the relational where clauses exist
• tests/Database/DatabaseEloquentHasOneOrManyPendingAttributesTest.php
  Copied from DatabaseEloquentHasOneOrManyWithAttributesTest.php and made sure only the relational where clauses exist
• tests/Database/DatabaseEloquentPendingAttributesTest.php
  Copied from DatabaseEloquentWithAttributesTest.php and made sure no where clauses

[taylorotwell]

I would rather have a flag on withAttributes that users can pass via a named argument.

[AndrewMast]

I reattempted this using a named argument in PR #55199.