[DESIGN] Extract logic for certificate renewal to its own separate method

[Mehdi-Bendriss]

Context of issue here #22 (comment)

[github-actions]

https://warthogs.atlassian.net/browse/DPE-4294

[juditnovak]

Adding the original comment for clarity:

We should consider extracting the logic of expiring into some method renew_certificates or similar and call it from here as well as from _on_certificate_expiring.
The reason being that this is now fine, but only because we are not making use of the event: CertificateExpiringEvent that's normally passed as an argument of _on_certificate_expiring.

We are not using the event by policy . It's not a co-incidence.

The charm was doing this before the particular modification that triggered this thread (see _set_tls_private_key()).

My question is: are there any risks implied by the current policy?