Generic certificate and key management

[fietser28]

In #210 a way for MQTT certificates is requested
In #215 firmware downloads are made, this should be secure (HTTPS download).
In the future more security/encryption related services are also likely (e.g. HTTPS / database connection in flows) these will also require certificate and key management.

Both of these require certificate/key management, this should be facilitated in a generic way.

I did not investigate available libraries or common practices for embedded / open source systems.