Add ability to configure `common_name` and `sans_dns`

According to the [Vault documentation](https://charmhub.io/tls-certificates-interface/docs/h-recommended-config-options), requirer applications must provide the following configuration keys

- `common_name`
- `sans_dns`
- `organization`
- `organizational_unit`
- `email_address`
- `country_name`
- `state_or_province_name`
- `locality_name`

This is not the the case for postgresql charm. This ticket acts as a feature request to allow the operator to configure such settings in the charm (at least `common_name` and `sans_dns` which are the most important ones)

postgresql charm should have the ability to configure such settings, like `common_name` and other config options necessary to manage certificates through Vault.

- Workaround for now: Allow Vault to issue certificates without taking into consideration the `common_name`
```
juju config vault pki_allow_any_name=true
```
This is a bit dangerous though