Add validation to Enabled/Disabled fields in ApplicationLayer

[tmjd]

Description

For PR author

• Tests for change.
• If changing pkg/apis/, run make gen-files
• If changing versions, run make gen-versions

For PR reviewers

A note for code reviewers - all pull requests must have the following:

• Milestone set according to targeted release.
• Appropriate labels:
  • kind/bug if this is a bugfix.
  • kind/enhancement if this is a a new feature.
  • enterprise if this PR applies to Calico Enterprise only.

[caseydavenport]

LGTM (you could also add the kubebuilder tags to the enum type definition instead and then the validation will be inherited everywhere those types are used. I don't think it is functionally different in this case, though)