Make maximum resident credential count configurable

[robin-nitrokey]

Previously, we estimated that we can handle 100 resident keys when returning the number of remaining resident keys in the credential management command.

This patch introduces a config option to set a maximum count of resident keys that is used to report the number of remaining resident keys and that is enforced when trying to create a new resident key.

[robin-nitrokey]

This is missing a fix for an issue that occurs when overwriting an existing resident keys, see: https://github.com/Nitrokey/fido-authenticator/pull/14