Added config for specific endpoint class

[ezimuel]

This PR adds the ability to specify $config on each endpoint class. Example as follows:

use Elastic\EnterpriseSearch\Client;

$client = new Client([
    'host' => 'http://localhost:3002'
]);
$workplaceRead = $client->workplaceSearch([
    'token' => '<AUTH_TOKEN for reading>'
]);
$workplaceWrite = $client->workplaceSearch([
    'token' => '<AUTH_TOKEN for writing>'
]);

This configuration allows having multiple client instances with different token permissions (e.g. for reading and writing).
The $config passed in the specific endpoint class (e.g. workplaceSearch($config)) will override the values specified in the Client.

You can even instantiate the Client without any $config, as follows:

use Elastic\EnterpriseSearch\Client;

$client = new Client();
$workplace = $client->workplaceSearch([
    'host' => 'http://localhost:3002',
    'token' => '<AUTH_TOKEN>'
]);

[picandocodigo]

LGTM!

[ezimuel]

Thanks @picandocodigo