Implement V1 of certificate_transfer

[saltiyazan]

Issue

The new version (V1) of the certificate_transfer_interface is incompatible of its V0 version. Applications that implement V1 of cert transfer won't be able to share their CAs with Traefik.

Solution

This change is to allow application the implement the V1 version of the interface to integrate with Traefik to provide their CAs that Traefik needs to trust.
Note: The solution might seem a bit hacky, I tried to follow the same solution with IngressPerAppV0 and IngressPerAppV1.

Context

Testing Instructions

• Deploy Traefik from this branch
• Deploy notary-k8s from this branch
• Deploy self-signed-certificates
• Integrate Notary and self-signed-certificates using the certificates interface
• Integrate Notary and Traefik using the receive-ca-cert-v1 interface
• Observe the CA file being created in the correct directory in the Traefik container
• Remove the integration and observe the file being removed

Upgrade Notes

[saltiyazan]

Hi, @sed-i @simskij
Could someone please take a look at this? happy to re-work it if needed.

[ca-scribner]

My uninformed first impression is that I dislike adding a new relation to handle a different version of the same interface. This feels like a change that should be transparent to the user if at all possible. Ideally, the existing receive-ca-cert relation should just work for both v0 and v1. Is that possible?

[dstathis]

If the interface is the same, we shouldn't use a new relation. The promise is supposed to be that any provider of an interface works with any consumer.

[saltiyazan]

V0 and V1 are not compatible. There was breaking change in the interface too not only the library.
Here you can see examples of the data expected in V0 and the data expected in V1.

Isn't it the same with Ingress V1 and V2?

[IbraAoad]

It's the same for ingress but handled differently not to require a new relation, traefik only has one relation for ingress and we do our checks internally within the charm code to validate if client is talking v0/v1

[saltiyazan]

I see your point thank you.
I'll update the PR.

[saltiyazan]

I updated the PR to follow the example of Ingress.
Could you please take a look @IbraAoad ?