Add delete support for Endpoint With Model Garden Deployment resource

[modular-magician]

This PR adds the necessary custom code templates and resource configuration fields to enable deletion support for the vertexai_endpoint_with_model_garden_deployment resource.

Specifically, this PR:

• Updates the EndpointWithModelGardenDeployment.yaml file with new fields to enable custom_delete.
• Adds a custom_delete template that undeploys the model deployed to the endpoint and deletes the endpoint.
• Updates post_create to read and set field values required to undeploy model from endpoint.
• Updates acceptance tests to verify that resource is deleted.
• Adds additional examples and acceptance tests demonstrating various use cases for the resource.

Vertex AI Model Garden service
Based on:

• ModelGardenService.deploy
• EndpointService.undeployModel
• EndpointService.deleteEndpoint

Addresses:

• resources google_vertex_ai_model and google_vertex_ai_endpoint_deployed_model not found terraform-provider-google#23217
• How do I enable models in the GCP model garden using terraform? terraform-provider-google#22478

vertexai: added `custom_delete` field to `google_vertex_ai_endpoint_with_model_garden_deployment` resource

Derived from GoogleCloudPlatform/magic-modules#14650