[8.x] Support created_at timestamps on upserts

[paras-malhotra]

As a follow-up to #34698, this PR adds support for updating created_at timestamps on upserts.

I've modified the signature of the upsert method to include an optional third argument to be an array of the columns to be updated. If not provided, all columns will be updated.

[taylorotwell]

Should we update DatabaseQueryBuilderTest to check the the raw SQL is correct using the new feature?

[paras-malhotra]

@taylorotwell, I've added DatabaseQueryBuilderTest tests for the new feature as well.

[taylorotwell]

I still don't really understand this method. Assume I have a Post database model. The table has slug (unique), title (string), and creation / update timestamps.

I want to bulk edit my post titles and upsert the posts... any new records should be inserted with a created at timestamp. I can't do this:

    $results = Post::upsert([
        ['slug' => 'foo', 'title' => 'foo updated'],
        ['slug' => 'bar', 'title' => 'bar updated'],
    ], 'slug');

What am I supposed to do?

[paras-malhotra]

@taylorotwell for that case, you'd have to do this:

$results = Post::upsert([
        ['slug' => 'foo', 'title' => 'foo updated'],
        ['slug' => 'bar', 'title' => 'bar updated'],
], 'slug', ['title']);

[mariomka]

Amazing work, I love it!

This PR also adds the possibility to define the value when a row is being updated, and it allows DB expressions. It is awesome, for example, you can add numbers:

Stats::upsert(
    [
        ['id' => 1, 'page_views' => 1],
        ['id' => 2, 'page_views' => 5],
    ],
    'id',
    [
        'page_views' => DB::raw('stats.page_views + excluded.page_views'),
    ]
);

It works for Postgres