[6.x] Allow callback scopes on Query Builder

[deleugpn]

Laravel Query Builder allow for some pretty dynamic code execution. We can pass a callback to select, addSelect, where, join and so on. However, we cannot pass a callback/closure to groupBy.
It's been a while since I've wanted to apply any dynamic scope to Query Builder and this time, instead of contributing just to groupBy I thought I'd give it a shot.

How it works

DB::table('my-table')->apply(new MyDesiredScope)->get();

This will send the Query Builder to the callable method (if MyDesiredScope is callable) and let it mutate the Query Builder. I've been doing a lot of this with where already and it works great:

Post::newQuery()->where(new PostsWithNewReplies);

I just had the need to do that at the groupBy clause instead and noticed I couldn't, hence the contribution to a broader audience.

Signature

apply will accept a single Scope, multiple arguments of scope or an array of Scopes. all cases are covered by tests.

Docs: laravel/docs#5533

[oliverquynh]

I think CreatedIn2017Scope is more suitable? or What about this?

$result = DB::table('posts')->apply((new CreatedInYearScope(2017)))->get();

[deleugpn]

I don't think that test class is relevant

[deleugpn]

I just learned about $builder->tap() and it covers most of what this PR tries to cover and is 🔥 .
I think we just need to document tap() as I couldn't find it on the docs. I'll follow up on that.

[taylorotwell]

OK, can you PR a documentation update for tap instead? It does seem to do basically what you want.