Switch default HTTP client from OkHttp to Vert.x

[manusa]

Description

Requires #5719

The default HTTP client implementation bundled with the Fabric8 Client is io.fabric8:kubernetes-httpclient-okhttp.

The main reason is that this is the client implementation that was used even before we provided an abstraction layer and multiple HTTP client implementations to choose from.
In addition, the Fabric8 MockWebServer was based on OkHttp MockWebServer.

OkHttp is the less performant of all the implementations.
Vert.x is the most compatible and versatile client implementation.
Vert.x is also used in many downstream projects, so it can be reused.

We need to switch the default bundled httpclient implementation from OkHttp to Vert.x.