Add __call() method to AlgoliaEngine

[tillkruss]

This allows calling methods like multipleQueries() on the Algolia search client easily.

I'm not sure if tests are needed for this. Please advise.

This PR allows calls like this:

User::searchableUsing()->multipleQueries();

// or

app(EngineManager::class)->engine()->multipleQueries($indices);

[nunomaduro]

The usage would be something like this, correct?

User::searchableUsing()->multipleQueries();

[tillkruss]

Correct, I updated the description.

[nunomaduro]

Alright, I do think that's a useful feature. @taylorotwell @driesvints Willing to add this?

@tillkruss If they agree, do you mind of adding some tests?

[tillkruss]

@nunomaduro: I wouldn't know what to test TBH without linking the tests to Algolia's SearchClient itself, which might change and I'd say it's up to the developer to test their call themselves, not this package. Do you have a suggestion?