[8.x] Fix fresh and refresh on pivots and morph pivots

[axlon]

Currently the Model::fresh() and Model::refresh() methods cannot be used on pivots, because they reference the model key directly. This PR alters these methods slightly to take advantage of the Model::setKeysForSaveQuery() method which already allows methods such as Model::delete() to work on pivots.

The following is now possible:

$model->relation->pivot->refresh()
$model->relation->pivot->fresh()

[jaulz]

@axlon I noticed that while using morphOne the following line removes the Pivot relation and does not reload Pivot relations:

framework/src/Illuminate/Database/Eloquent/Model.php

Lines 1466 to 1469 in 0c4d197

	$this->load(collect($this->relations)->reject(function ($relation) {
	return $relation instanceof Pivot
	|| (is_object($relation) && in_array(AsPivot::class, class_uses_recursive($relation), true));
	})->keys()->all());

Do you think this PR can also help to reload Pivot relations when calling reload on an instance?

[axlon]

@jaulz From looking at this at first glance I don't think the snippet you shared is related to this PR. This PR and its followup #35193 only deal with calling fresh en refresh from the point of view of the pivot, e.g.:

$party = Party::find(1);
$invitee = $party->invitees[0];
$invite = $invitee->pivot;
$invite->refresh(); // <-- this was fixed by the PRs

It seems to me pivot relations are skipped on purpose, but I don't know why