Support for postgresql_roles data source

[nb-ccd]

Hi, would there be any interest in providing a data source for roles?

This would be useful to me because I'm making a tool which uses Pulumi (and this provider under the hood) to provision databases for an application. This can either be used to restore a database from a snapshot or to create a new one, and so roles may or may not already exist.

For my case just providing a list of role names with no other attributes would work completely fine, so just output from SELECT rolname FROM pg_roles; or the same with optional WHERE rolname LIKE filter options.

I'm not very familiar with either go or Terraform providers but if this seems worthwhile I would be happy to take a look at the weekend and see if I can get a dev env up and running to take a stab at a PR.

[mt-concordium]

That would be super useful! Users management is the main feature of the provider, so would be great to get support for managing these extended.