fix: Access kv secret using secret ID

[saltiyazan]

Description

fixes https://github.com/canonical/vault-operator/issues/243

Not to lose the secret://modeluuid/standard_secret_id format of the secret ID which is required to access the secret in cross model relations we get the secret with the secret ID that was initially returned by add_secret and initially stored in the relation databag.

Bug reported in Juju https://bugs.launchpad.net/juju/+bug/2075153

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

[DanielArndt]

I'm going to be a jerk and disable auto-merge here, because I think my comment on the other PR is worth considering. It could always come as a follow-up, but I figured it would be worthwhile for you to consider before merging, and I know I was a little late on the review.