Implement ListClients and DeleteClient in the core provider

[hug-dev]

Implemented similarly as other operations (before the provider level), but they would require admin privilege.

Provide trait method:

    fn list_clients(
        &self,
        op: list_clients::Operation,
    ) -> Result<list_clients::Result> {

In the future this would also contain as parameter the authentication type to only return clients of that type.

The operation would be implemented in the core provider.

For ListClients to do something similar than ListKeys: have the list_clients method be implemented by all providers to use the key info manager implementation they are using to return a list of client. The Core provider would call list_clients on all of its Provide implementations (which it already has) and merge them.
ListClients should only be callable on the Core Provider, see #310 .

For DeleteClient, we can use a mix of existing methods: list_keys on the application name given as parameter and then psa_destroy_key on each one of them.

Testing

We can test this in the all-providers multitenancy tests. Making sure that the admin can see all clients and that he can delete all keys of the other one 💯