refactor: use TLS library v4

[gruyaume]

Description

Bump the TLS library to V4.

Addition of certificate helper methods

The addition of src/certificates.py is because the TLS library V4 does not provide the generate_certificate, generate_private_key, and generate_ca helpers as those were specific to this self-signed certificates charm.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have bumped the version of the library

[saltiyazan]

I went through it and it looks really good. I have a few comments some of which might not be directly on the PR but I think they could be included here.

[kayra1]

one small comment

[saltiyazan]

Looks good. I created tickets for the comments that would not be addressed as part of this PR.
For the comment on get_certificate_extensions I prefer a better name, but I'll leave it up to you to resolve the comment either accepting or rejecting the new suggestion.