[REQ][php] Allow consumers to configure http version

Is your feature request related to a problem? Please describe.

There is no way to turn on http/2 in php client without messing with templates atm

Guzzle supports it like this:

use GuzzleHttp\Client;

$client = new Client();
$client->get('https://google.com/', [
    'version' => 2.0,
    'debug' => true,
]);

Describe the solution you'd like

It looks like a good idea to extend Configuration.php with httpVersion/getHttpVersion/setHttpVersion.
If it's expicitly set by the client consumer, than we can use it in createHttpClientOptions and call it a day

Describe alternatives you've considered

We can also just set version 2.0 by default, but this will probably lead to lots of problems since http/2 support in Guzzle is far from being ideal

Start with Configuration.mustache to inspect the proposed httpVersion, getHttpVersion, and setHttpVersion additions, then trace createHttpClientOptions in api.mustache. Done means a generated PHP client lets consumers configure the HTTP version and passes that value into the Guzzle request options.