Make responses PSR-7 Compliant

[polyfractal]

Making the responses coming out of of Elasticsearch-PHP PSR-7 compliant shouldn't be too difficult, and I think it can be done in a way that doesn't violate the "no opinion" goal of all low-level ES clients (and without breaking bwc)

I'm unsure if the requests can easily be made compliant, but we can explore that avenue as well

[jeskew]

👍 to having requests be PSR-7 compliant (and for having a method that would create a request object and return it to you). That would make it easier to support more kinds of authentication.

[sagikazarmark]

Speaking of PSR-7: several PHP HTTP components became stable over the past few weeks:

• HTTPlug: a PSR-7 HTTP Client abstraction layer
• Message: several message helper components
• Message Factory: interfaces for PSR-7 Message Factories

[ezimuel]

I'm working on a new elasticsearch-php client that will be released for the next major (8.0). This will support PSR-7, PSR-18.