Handle configuration of backends used by applications

[sosthene-nitrokey]

Opcard and the secrets app will soon have the ability to use the se050 backend.

For that, we will use the configuration mechanism from #7

Given how #7 and the backend configuration works, the configuration can only be taken into account after a reboot.

There are 2 issues:

1. The state of the affected app will need a factory reset.
2. We must prevent the user from mistakenly thinking that the config has been applied without a power-cycle.

For (2), I think that triggering a power cycle from the admin app after any config has been changed would make sense to me.

For (1) I initially thought that we could add the current backend configuration as a data parameter that apps have access to. Then if the backend config has changed they must notice it when loading their state and factory-reset themselves. But this faces the issue that at that point they don't have access to the other backend anymore.

We could implement the factory-reset mechanism through an extension and allow a "priviledged" app to factory-reset other applications, and give that responsibility to the admin-app.
Such a "priviledged" extension is already being worked on in #6 and https://github.com/Nitrokey/trussed-se050-backend/blob/core-apis/src/manage.rs.