[Feat] Implements raft backend (HA)

[gruyaume]

Description

Implementation of HA leveraging the Raft backend. The leader initializes Vault and as unit gets added to the application, they look at the existing unit addresses from the peer relation data and use those in their configuration file to get added to the raft cluster.

Reference

• https://developer.hashicorp.com/vault/docs/configuration/storage/raft

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

[ghislainbourgeois]

I have a small question remaining