Implement new cloud SQL API switchover

[lychgoogle]

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Description

We have new API switchover on cloud SQL (API doc) https://cloud.google.com/sql/docs/mysql/admin-api/rest/v1/instances/switchover , and we want to support them in terraform.

New or Affected Resource(s)

• google_sql_database_instance

Potential Terraform Configuration

Suppose we have "original-primary" and "original-replica", then we make following change to the replica to trigger switchover.

resource "google_sql_database_instance" "original-replica" {
  provider = google-beta
  name = "original-replica"
  project = "my-project"
-  instance_type = "READ_REPLICA_INSTANCE"
+  instance_type = "CLOUD_SQL_INSTANCE" 
+  replica_names = ["original-primary"]
# irrelevant fields are omitted.
}

References

No response

b/359238666

[lychgoogle]

Addition on assignee: I'd like to work it myself, but not sure how to make me as an assignee.

[lychgoogle]

main PR merged, so my job seems done.