Improve config

[robin-nitrokey]

• implement Default::default or add constructor
• require max_resident_credential_count

[robin-nitrokey]

Also, max_resident_credential_count is not taken into account properly for the estimated remaining discoverable credentials in the credentials metadata response.

→ #105

[robin-nitrokey]

A constructor has been added in trussed-dev#65. The max resident credential count has been reworked since anyway.