Make backends configurable

[riccardomc]

We decided to simplify the architecture of the operator and reduce the cardinality of backends per operator instance to only one. We will also limit the scope to one target namespace. This will greatly simplify configuration and minimize unwanted behavior.

This PR introduces the behavior for the operator to check for an OPERATOR_CONFIG environment variable that contains JSON formatted configuration data based on the following struct:

type BackendConfig struct {
	Name       string
	Type       string
	Parameters map[string]string
}

The Parameter field is free-form and up to the specific backend type to specialize. The following is an example for the AWS Secrets Manager backend:

{
  "Name": "asm-example",
  "Type": "asm",
  "Parameters": {
     "accessKeyID": "$AWS_ACCESS_KEY_ID",
      "region": "$AWS_DEFAULT_REGION",
      "secretAccessKey": "$AWS_SECRET_ACCESS_KEY"
    }
}

An example is provided on how to use a Secret to populate the OPERATOR_CONFIG environment variable for the AWS Secret Manager Backend.