Propose required defaults for the NATS-CR

[grischperl]

Description
Follow up to #38

As defaulting CR values using the OpenAPI v3 validation schema is possible and desired, this issues aim is to propose which values should have a default and what that default is.

Acceptance

• Which NATS CR fields should have a default value: every field should have a default value
  Concerns about immutable fields: Installing the CR from lifecycle manager is optional, which means the user creates the CR themselves. Still, there will be a feature request for the user to be able to review the CR before it gets installed into the cluster from the lifecycle manager.
• What is the default value for these fields: [POC] upgrading from kyma 2.x to NATS module [TB: 2d] #33 (comment)
• After discussing with the team: Implement the defaults / discuss with person doing the validation schema