[8.x] Extract attribute getter for performInsert

[donnysim]

With models we disabled $guarded or $fillable and instead opted for a $columns concept where we define all database columns on the model e.g.

protected static array $columns = [
    'id',
    'email',
    'password',
    'name',
    'last_name',
    'remember_token',
    'created_at',
    'updated_at',
    'deleted_at',
    'email_verified_at',
];

This allows us to have a column reference list without having to dig into the database, and also we use this list to filter attributes before saving to avoid some unexpected missing column exceptions when paired with custom properties (after joins etc.). For this to fully work we need to override performInsert where instead of $this->getAttributes() we do

$columns = static::columns();
$attributes = empty($columns) ? $this->getAttributes() : Arr::only($this->getAttributes(), $columns);

While we have no problems overriding this, there still is a possibility of a breaking change to occur without us knowing and it'd be safer to manage if the attribute extraction is separated. This also helps us with computed columns where it exists in database as a column but is readonly, in that case we mark it as a comment in the column list.