Enable External Infinispan Feature

[pruivo]

• New option to enable external infinispan feature (disabled by default)
• Added to the nightly runs to get some numbers

It has been a while since I looked at those GH action files. Please double-check if I did it right. I tested a deployment with KC_EXTERNAL_INFINISPAN=true and confirmed no embedded caches were present in Keycloak Servers.

[pruivo]

I didn't add the functional tests to the GitHub actions. I forgot. It can be merged and hope it does not break horreum :)

cc @ahus1 @ryanemerson

[ryanemerson]

Thanks @pruivo