[PHP] Add support for async requests

[baartosz]

Description

Generated clients are already using guzzle6 out of the box on 2.3.0 branch and guzzle supports async requests. It is easy task now to allow generated client to send async requests. I would suggest adding separate $api->xxxAsync() and $api->xxxAsyncWithHttpInfo() methods for this.

Swagger-codegen version

2.3.0+

Swagger declaration file content or url

Command line used for generation

--library=none or something

Steps to reproduce

Related issues

#5190
#1482

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