gcp_sql_instance: support "edition" field

##### SUMMARY
Support the "edition" field, either "enterprise" or "enterprise-plus".

##### ISSUE TYPE
- Feature Idea

##### COMPONENT NAME
google.cloud.gcp_sql_instance

##### ADDITIONAL INFORMATION
As it stands, e.g. there is no way to create a Postgres 18 database with Enterprise. Minimal Enterprise Plus instances are much more expensive than minimal Enterprise instances (~$180/mo vs. ~$7-$10/mo), so this limits the usefulness of this module.

See https://docs.cloud.google.com/sdk/gcloud/reference/sql/instances/create#--edition

```yaml
- name: Create DB instance
google.cloud.gcp_sql_instance:
name: "{{ cloud_database_name }}"
database_version: "{{ cloudsql_database_version }}"
settings:
edition: enterprise
region: "{{ cloudsql_region }}"
project: "{{ gcloud_project }}"
state: present
```

Start with the google.cloud.gcp_sql_instance component and compare its settings handling with the Google Cloud SDK --edition reference linked in the issue. Support enterprise and enterprise-plus as shown in the YAML example, then verify that the example can create a PostgreSQL instance with the selected edition.

Written by the indexing model from the issue text.