Immutable fields in Spec

[nmvk]

Is your feature request related to a problem?
Currently many APIs have fields which are valid during creation but cannot be modified later. This can potentially create confusion as the resource would not be updated.

Example -

https://docs.aws.amazon.com/cli/latest/reference/elasticache/create-replication-group.html

In this ElastiCache API, --transit-encryption-enabled , --at-rest-encryption-enabled are currently immutable.

Describe the solution you'd like
Validate if an immutable spec field is modified, then set appropriate condition.

Describe alternatives you've considered
Webhook to validate the immutable fields.

[a-hilaly]

I believe this is done in aws-controllers-k8s/code-generator#77 and aws-controllers-k8s/runtime#17.

/close