Add timers for certificate expiration check (A03)

[klemmpnx]

Add timers for the renewal of charging station certificates (use case A03). Oriented at OCPP1.6 implementation.

Note: requires update of libevse-security to correctly return remaining valid days (see https://github.com/EVerest/libevse-security/pull/14/files)

Tested in https://github.com/PionixInternal/ocpp-testing/pull/34

[klemmpnx]

One concern of mine: I added the initialization of the timers to both the handle_boot_notification_response as well the connected_callback of the websocket (here conditioned though if there was a "offline" time). The latter is necessary I think because the timers need to be stopped at any disconnect.
However, I am not sure when exactly the this->registration_status == RegistrationStatusEnum::Accepted condition is fulfilled and it might be more useful to just start the timers every time (unconditioned) in the connected_callback instead of the boot_notification_response?

[Pietfried]

One concern of mine: I added the initialization of the timers to both the handle_boot_notification_response as well the connected_callback of the websocket (here conditioned though if there was a "offline" time). The latter is necessary I think because the timers need to be stopped at any disconnect. However, I am not sure when exactly the this->registration_status == RegistrationStatusEnum::Accepted condition is fulfilled and it might be more useful to just start the timers every time (unconditioned) in the connected_callback instead of the boot_notification_response?

It is not allowed to send this message (SignCertificate.req) to the CSMS before the BootNotification.req has been confirmed with a BootNotification.conf(Accepted) from the CSMS (== RegistrationStatus::Accepted), so the implementation is just fine

[Pietfried]

Please check again in which situations we should request a V2G or client certificate and only set the timers if required