Support to specify an external Keycloak instance for SSO

[tdkl]

Is your feature request related to a problem? Please describe.
Going through the documentation for the ArgoCD Operator for an OpenShift deployment (https://argocd-operator.readthedocs.io/en/latest/usage/keycloak/openshift/), I wasn't able to find a specific ability to configure an external Keycloak instance to be used for OIDC. The configuration examples consist only of deploying a new and separate Keycloak instance, which isn't even persistent, which I found - interesting. If an organization already has a Keycloak (or other) instance, I don't find it really useful, to create one for every single implementation.

Describe the solution you'd like
Ability to configure an external Keycloak instance to be used for SSO login, like stated on regular ArgoCD documentation: https://argo-cd.readthedocs.io/en/stable/operator-manual/user-management/keycloak/, which isn't an operator deployed instance.