driver/kubernetes: add ownerrefs driver option for owner references

[abhay1999]

Fixes #2626

Add support for setting Kubernetes ownerReferences on the builder
Deployment/StatefulSet via a new ownerrefs driver option.

This allows the builder to be automatically garbage-collected by
Kubernetes when its owning resource (e.g. an ephemeral GitHub Actions
runner) is deleted, removing the need to manually patch the Deployment
after creation.

Usage:

docker buildx create \
  --driver kubernetes \
  --driver-opt "ownerrefs=apiVersion=actions.github.com/v1alpha1,kind=EphemeralRunner,name=runner-xyz,uid=<uid>,controller=true,blockOwnerDeletion=false"

Multiple owner references can be specified by separating them with
semicolons:

--driver-opt "ownerrefs=apiVersion=v1,kind=Pod,name=pod1,uid=uid-1;apiVersion=v1,kind=Pod,name=pod2,uid=uid-2"

Each owner reference supports the following fields:
apiVersion, kind, name, uid, controller, blockOwnerDeletion

The implementation follows the same parsing pattern as the existing
tolerations driver option (semicolon-separated entries, each with
comma-separated key=value fields).

Changes:

• driver/kubernetes/manifest/manifest.go: add OwnerReferences field
  to DeploymentOpt and set it on the Deployment/StatefulSet ObjectMeta
• driver/kubernetes/factory.go: parse the ownerrefs driver option
• driver/kubernetes/factory_test.go: add tests for single owner ref,
  multiple owner refs, and invalid inputs

Signed-off-by: abhay1999 abhaychaurasiya19@gmail.com

[AkihiroSuda]

To avoid inflation of the switch{}, can we rather support an arbitrary yq/jq expression to patch the manfiest?