Managing unique wallet address keys

[mkurapov]

Context

Based on the discussion in #2795, we noticed that Rafiki lets uploading duplicate keys to a wallet address as there is no unique constraints. This might lead to confusing behavior for validating keys.

We should prevent unique keys from being uploaded, particularly by kid, since it needs to be clear which key (and corresponding key value) the Open Payments client is using in their requests.

Todos

• Deleting existing duplicate keys: Remove active duplicates, keep the most recent active key
• Prevent unique keys from being uploaded by adding a unique constraint on (walletAddressId, kid). We should check for the UniqueConstraintError in the create method to give a proper error message back in the API response