Replace existing CLI args with a mounted operator configuration

[unmarshall]

How to categorize this issue?

/area control-plane
/area usability
/area dev-productivity
/kind enhancement
/kind api-change

What would you like to be added:

Why is this needed:
etcd-druid currently has a lot of CLI args that are individually exposed and needs to be explicity specified when configuring command line arguments in the deployment resource. This creates a challenge to support downgrade when new CLI flags are added to the newer version. One has to explicitly use pflag library and its capability to ignore CLI args that it does not understand.
Having a well defined API type for configuration allows the following:

• One file where all the operator configuration is visible
• One can register the scheme and with it one can register the defaulting and validating functions. This ensures that the rest of the code does not have to deal with providing defaults and validate it at different places.

This is a general pattern that has also been seen in both gardener and other k8s community projects.