✨ Add ExistingLaunchTemplate (BYO) support for EKS managed node groups

[AmitSahastra]

What this PR does

Adds a new existingLaunchTemplate field to AWSManagedMachinePoolSpec that allows users to reference a pre-existing (BYO) AWS launch template by ID or name for EKS managed node groups, instead of having CAPA create and manage one.

This enables use cases where organizations manage launch templates externally (e.g. via Terraform, CloudFormation, or the AWS console) and want CAPA to use them as-is.

Fixes #5896

Key changes

API (v1beta1 + v1beta2):

• New ExistingLaunchTemplateRef type with id, name, and version fields
• New existingLaunchTemplate field on AWSManagedMachinePoolSpec, mutually exclusive with awsLaunchTemplate

Controller:

• Resolves BYO launch template by ID or name during reconciliation
• Sets launch template ID and version in status
• Skips launch template deletion for BYO templates

Webhook validation:

• Mutual exclusivity between existingLaunchTemplate and awsLaunchTemplate
• Requires exactly one of id or name
• Requires version
• id and name are immutable; version is intentionally mutable (to allow rolling out new LT versions)
• diskSize and instanceType are rejected when any launch template is specified

Nodegroup service:

• BYO launch template is passed to CreateNodegroup and UpdateNodegroupVersion
• Symbolic version handling ($Latest, $Default) to avoid endless reconcile loops
• Guard against nil Version/ReleaseVersion on failed nodegroups

Tests:

• Webhook: create and update validation tests for all BYO launch template scenarios
• Nodegroup: unit tests for isSymbolicLaunchTemplateVersion helper

How to test

1. Create an EC2 launch template in AWS
2. Reference it in an AWSManagedMachinePool spec:

spec:
  existingLaunchTemplate:
    id: lt-xxxxxxxxxxxx
    version: "1"

3. The node group should be created using the referenced launch template
4. CAPA should NOT delete the launch template on pool deletion

Release note:

  Add BYO launch template support for EKS managed node groups

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[k8s-ci-robot]

Hi @AmitSahastra. Thanks for your PR.

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[AmitSahastra]

Hi @fiunchinho @serngawy the bot suggested I consider joining the kubernetes-sigs org given my contributions. Would either of you be willing to sponsor me for membership? I'm happy to open the formal request at https://github.com/kubernetes/org/issues/new?template=membership.md once I have two sponsors confirmed. Thanks!

[serngawy]

/ok-to-test

[AmitSahastra]

/retest