APIRule reconcile function refactoring

[strekm]

Description

Introduction of APIRule v2alpha1 added complexity to main reconcile function that needs to be addressed before adding extAuth handler.

ACs:

• removed complexity from reconcile function
• make implementation easy to extend with another handler
• make reconcile function version aware
• Create follow up issues

Additional info

• Configuration for immutable fields can be achieved by using CEL validation:

  // +kubebuilder:validation:XValidation:rule="self == oldSelf",message="Value is immutable"

  This might be useful to support conversion from v1beta2 to 1 without need for preserveUnknownFields.

DoD:

• Provide unit and integration tests. -> integration tests in follow up, but also migration integration tests exist that cover the implementation.
• Provide documentation.
• Verify if the solution works for both open-source Kyma and SAP BTP, Kyma runtime.
  - [ ] If you changed the resource limits, explain why it was needed.
• Verify that your contributions don't decrease code coverage. If they do, explain why this is the case.
  => code coverage decreased from 61,1% to 59,2% but this is also due to open todos like for v2alpha1 validation, because some of the code already exists to match the interfaces, but the real implementation and tests will be done in a follow up.
  - [ ] Add release notes.

Attachments
part of: #939

PR

• Refactor APIRule reconciliation loop #1140

Follow ups

• Switch processors implementation in v2alpha1 reconciliation #1147
• Switch status implementation in v2alpha1 reconciliation #1148
• Improve v2alpha1 validation  #1149
• Integration tests for v2alpha1 APIRule #1151