Update Keycloak Test Module to support code flow tests

[sberyozkin]

This PR does a few minor improvements to the newly introduced Keycloak test module nicely done by @cemnura :

• Customizes realm, client ids, makes sure it can support both service and web-app tests, work over plain HTTP
• Updated the docs
• Updated the dev code flow tests in extensions/oidc/deployment to use it

[gastaldi]

LGTM, just some minor changes

[geoand]

@sberyozkin Hi,

can you please squash the commits?

Thanks

[sberyozkin]

@geoand Hi Georgios, oh sorry, will do, by the way, I wanted to give it a quick try and see if this test module can also be used to test quarkus-oidc-client, so I'll look into it a bit and make sure the commits are squashed as well, thanks

[geoand]

Cool, thanks for the info

[sberyozkin]

@gastaldi @pedroigor Just pushed another update - where I switched oidc-client-filter/deployment to use the new test module and also did a minor doc update.
@gsmet thanks very much for your help with github related issues :-) (for the record, I've messed up the history by accidentally rebasing while pulling George's updates) - please cancel it if it is too much for CI at this point - I'll restart later - not an urgent PR

[sberyozkin]

OK, it is all green so I'll go ahead with the merge :-)