Use Elasticsearch contract instead of concrete class in constructor

[floristenhove]

For service containers to correctly handle dependency injection we should pass the interface instead of the concrete implementation of the Elasticsearch PHP Client.

[boserup]

This is also useful when testing. See elastic/elasticsearch-php#1227

[bram-pkg]

I appreciate this is useful, but I believe this would be a breaking change.

People that dependency inject a bound version of the "Client" class will have to update the container binding to bind this interface to a client implementation now.

An example in Laravel would be:

app()->bind(Client::class, function (): Client {
    // create configured client here
});

// this would now break, if Builder is changed to have `ClientInterface` as the typehint.
app(Builder::class)->index('my-index')->search();