refactor: Remove secret usage from Vault PKI implementation for CA CSR

[DanielArndt]

Part of the solution for: #415

Currently, we store the CSR in a secret so that we can compare the certificate that is returned with the one we sent. However, we already store this CSR in the relation databag, and we are only ever requesting a single certificate.

Removing the dependence on secrets will simplify the solution for #415 because we won’t need to consider as many cases in the event that adding or updating the secret fails. Furthermore, it simplifies our code.

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

[gruyaume]

Can you improve the PR description, only looking at the code changes does not make it obvious why we are doing them.

[DanielArndt]

@gruyaume yeah, sorry, I added it to the Jira ticket but forgot to move it here.