feat(container): add endTimeBehavior to GKE maintenance exclusion opt…

[Bhavikshah406]

Description of your changes

• This PR exposes GKE end_time_behavior on maintenance_policy.maintenance_exclusion.exclusion_options, matching the HashiCorp google_container_cluster Terraform resource (end_time_behavior → optional string, e.g. UNTIL_END_OF_SUPPORT).

Changes:

• Add endTimeBehavior to ExclusionOptions* in cluster APIs: container/v1beta1, container/v1beta2, and namespaced/container/v1beta1 (with tf:"end_time_behavior,omitempty").
• Update generated deepcopy for those types.
• Extend Cluster CRD OpenAPI (container.gcp.upbound.io_clusters, container.gcp.m.upbound.io_clusters) so spec / status shapes accept endTimeBehavior under exclusionOptions.
• Update config/schema.json (resource block + computed maintenance_policy shape) and config/provider-metadata.yaml for the new attribute.
  Note: Upstream Terraform already supports this field; the provider’s checked-in schema had not yet included it. This aligns the Upjet types and CRDs with Terraform’s schema. A follow-up could bump TERRAFORM_PROVIDER_VERSION and run a full make generate to regenerate everything from the provider, if maintainers prefer that workflow.

Fixes #903

I have:

• Read and followed Crossplane's [contribution process].
• Run make reviewable to ensure this PR is ready for review.

How has this code been tested

• go build ./apis/...
• Confirmed staged diff is limited to the intended files (APIs, deepcopy, CRDs, config/schema.json, config/provider-metadata.yaml) with no accidental large deletions in schema/metadata.