Add support for adding comments to PostgreSQL roles

[kankou-aliaksei]

Summary

This PR adds support for attaching comments to PostgreSQL roles, allowing users to document the purpose of each role within the database. This improves clarity and supports compliance requirements.

Use Case

Teams managing PostgreSQL roles across environments can add comments to provide context or document role policies, aiding in audits and compliance.

Example

resource "postgresql_role" "my_role" {
  name     = "my_role"
  login    = true
  password = "mypass"
  comment  = "Role created for application access (JIRA: PROJ-1234)"
}