Implement linode_database_postgresql_v2 data source

[lgarber-akamai]

📝 Description

This pull request introduces a new linode_database_postgresql_v2 data source with schema matching the schema of the linode_database_postgresql_v2 resource.

✔️ How to Test

The following test steps assume you have pulled down this PR locally.

Integration Testing

make PKG_NAME=linode/databasepostgresqlv2 int-test

Manual Testing

1. Using a terraform-provider-linode sandbox environment (e.g. dx-devenv), apply the following configuration:

output "datasource_result" {
  value = data.linode_database_postgresql_v2.foobar
  sensitive = true
}

data "linode_database_postgresql_v2" "foobar" {
  id = linode_database_postgresql_v2.foobar.id
}

resource "linode_database_postgresql_v2" "foobar" {
  label = "mydatabase"
  engine_id = "postgresql/16"
  region = "us-mia"
  type = "g6-nanode-1"

  allow_list = ["10.0.0.3/32"]
  cluster_size = 3

  updates = {
    duration = 4
    frequency = "weekly"
    hour_of_day = 22
    day_of_week = 2
  }
}

2. Ensure the output accurate reflects the database's information.

[ezilber-akamai]

Tested locally and everything is working properly. LGTM

[jriddle-linode]

LGTM and works locally