Release 8.0.0 alpha

This is an alpha release of 8.0.0. This new major version of elasticsearch-php contains a brand new implementation compared with 7.x.
It supports PSR-7 for HTTP messages and PSR-18 for HTTP client communications. We used the elastic-transport-php library for HTTP communications.

We tried to reduce the BC breaks as much as possible with 7.x but there are some (big) differences:

• we changed the namespace, now everything is under Elastic\Elasticsearch;
• we changed the Exception model, using the namespace Elastic\Elasticsearch\Exception. All the exceptions extends the
  ElasticsearchException interface, as in 7.x;
• we changed the response type of each endpoints using an Elasticsearch response class.
  This class wraps a a PSR-7 response allowing the access of the body response
  as array or object. This means you can access the API response as in 7.x, no BC break here! 👼

You can have a look at the BREAKING_CHANGES file for more information.

We are still working on the new documentation for 8.0, in the meantime you can have a look at the README where we reported a quick start guide.