pg_hba_rules: An helper class to create various pg_hba_rule resource

This helper class allows on to create several pg_hba_rule resource at once.
It is necessary when using 100% data-driven architecture.

With a manifest like the following :

```puppet
include ::postgresql::globals
include ::postgresql::server
include ::postgresql::server::pg_hba_rules
```

One could write the following hiera file

```yaml
---
postresql::server:pg_hba_rules::pg_hba_rules:
  puppetdb:
    type: host
    database: puppetdb
    user: puppetdb
  gitlab:
    type: host
    database: gitlab
    user: gitlab
```