feat(gcp): add Cloud SQL instance resource

[Amit2465]

Description

Add GCP Cloud SQL instance (cloud-sql-instance) as a new supported cloud-nuke resource.

How it works

List: Paginates through all Cloud SQL instances in the project. Instances in transient states (PENDING_CREATE, PENDING_DELETE, MAINTENANCE, ONLINE_MAINTENANCE, REPAIRING), instances with deletion protection enabled, and externally managed instances (EXTERNAL backend / ON_PREMISES_INSTANCE type) are skipped. Read replicas and read pool nodes are returned before primary instances to ensure correct deletion order, the Cloud SQL API rejects deletion of a primary that still has active replicas.

Delete: Sends a delete request for each instance and polls the returned long-running operation until it reaches DONE. Already-deleted instances (404) are handled gracefully.

Identifier format: Composite — {project}/{instance}

Test output

=== RUN   TestCloudSQLInstances_ResourceName
--- PASS: TestCloudSQLInstances_ResourceName (0.00s)
=== RUN   TestCloudSQLInstances_MaxBatchSize
--- PASS: TestCloudSQLInstances_MaxBatchSize (0.00s)
PASS
ok  	github.com/gruntwork-io/cloud-nuke/gcp/resources	0.029s

Pre-commit checks:

• gofmt — no issues
• golangci-lint run ./... — 0 issues
• go build ./... — clean
• go vet ./... — clean

Manually verified against a real GCP project:

• inspect-gcp correctly lists Cloud SQL instances
• Deletion completes successfully with operation polling

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• Include release notes. If this PR is backward incompatible, include a migration guide.
• Attention Grunts - if this PR adds support for a new resource, ensure the nuke_sandbox and nuke_phxdevops jobs in .circleci/config.yml have been updated with appropriate exclusions (either directly in the job or via the .circleci/nuke_config.yml file) to prevent nuking IAM roles, groups, resources, etc that are important for the test accounts. (Note: nuke_sandbox and nuke_phxdevops only run aws, so no exclusions are needed for this new GCP resource.)

Release Notes (draft)

Added GCP Cloud SQL instance (cloud-sql-instance) as a new supported resource type.

Migration Guide

This PR introduces cloud-sql-instance as a new GCP resource type. Since cloud-nuke automatically includes all registered resource types, Cloud SQL instances will be nuked by default if no config file is provided.
To opt out, add the following to your cloud-nuke.yml config file:

GcpCloudSQLInstance:
  exclude:
    names_regex:
      - ".*"

[james00012]

Looks good overall. A few items:

1. Global vs regional registration — Cloud SQL instances are regional but registered in getRegisteredGlobalResources(). Is this intentional? Could instances in some regions get missed?
2. Missing t.Parallel() in both tests in cloudsql_test.go.
3. Use go-commons/errors — listCloudSQLInstances, deleteCloudSQLInstance, and waitForCloudSQLOperation use fmt.Errorf. Project convention is errors.WithStackTrace().
4. Replica ordering test: The replicas-before-primaries ordering is a correctness invariant for deletion. Worth adding a unit test for it.

[Amit2465]

Thanks for the review, all fixed.

• Global registration is intentional — Instances.List is project-scoped and returns all
  instances across all regions in one call, same as all other GCP resources here.
• Wrapped all fmt.Errorf returns with goerrors.WithStackTrace (aliased to keep stdlib
  errors available for errors.As).
• Added t.Parallel() to all tests and introduced a CloudSQLInstancesAPI interface for
  mocking to add TestCloudSQLInstances_ReplicaOrdering verifying replicas always precede
  primaries. this is correct