Add ability to configure TTL, Retry threshold parameters for leader election in Patroni

Hello,

During investigation of leader election issues today on Mattermost, we ran into a situation where we wanted to configure settings on Patroni that are not exposed through Juju configuration. I am opening this ticket to request these configuration parameters be made accessible through Juju. This is important to IS to avoid manual work on setting these parameters in DB environments

## Expected behavior

For Patroni config from https://patroni.readthedocs.io/en/latest/dynamic_configuration.html,

Ability to configure retry timeout by running `juju config postgresql patroni_retry_timeout=60`, etc.

## Actual behavior

We need to manually retrieve patroni password from `/var/snap/charmed-postgresql/current/etc/patroni/patroni.yaml` and curl to the patroni endpoint to set this value

```
# Without a relation between PG and any TLS operator.
curl -X PATCH -u user:password http://UNIT-IP:8008/config -d '{"retry_timeout": 60, "ttl": 130}'

# Or with a relation between PG and any TLS operator.
curl -k -X PATCH -u user:password https://UNIT-IP:8008/config -d '{"retry_timeout": 60, "ttl": 130}'
```

## Versions
Environment: dbaas-prod-airbyte-db on PS6 production
Juju CLI: 3.6.9

Juju agent: 3.6.9

Charm revision: 553 (14/stable)

## Additional context
Discussed [on mattermost here](https://chat.canonical.com/canonical/pl/by3grtibptruukmix935k39q9r)