feat: Add create_webhook attribute to buildkite_pipeline resource

[lox]

Description

Adds the ability to automatically create a GitHub webhook when creating a pipeline, leveraging the GitHub App integration. This eliminates the need for users to have repo admin access or manually create webhooks.

Context

Customer feedback requested IaC support for automatic webhook creation. This depends on a corresponding backend change to the Buildkite GraphQL API that adds the createWebhook input field to the pipelineCreate mutation.

Note: This PR is a draft until the backend changes are deployed.

Changes

buildkite/resource_pipeline.go

• Add create_webhook input attribute (optional, defaults to false)
• Add webhook_created computed attribute
• Pass createWebhook to the GraphQL mutation
• Preserve values through Update lifecycle

buildkite/graphql/pipeline.graphql

• Add createWebhook to the mutation input
• Request webhookCreated in the response

schema.graphql

• Add createWebhook input field and webhookCreated output field (will be regenerated from API once backend is deployed)

examples/resources/buildkite_pipeline/resource.tf

• Add example showing create_webhook = true usage

Verification

• go build passes
• End-to-end testing pending backend deployment

Example

resource "buildkite_pipeline" "example" {
  name           = "my-pipeline"
  repository     = "git@github.com:org/repo.git"
  cluster_id     = data.buildkite_cluster.default.id
  create_webhook = true
}

Before Merging

1. Backend changes must be deployed
2. Regenerate schema: make schema generate
3. Run acceptance tests

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