Make SECC certificate renewal threshold configurable

[Pietfried]

OCPP Version

OCPP1.6, OCPP2.1, OCPP2.0.1

Describe the problem

A03.FR.02 of OCPP requires to

The Charging Station SHALL generate a new public / private key pair using
one of the key generation functions described in Section 4.2.1.3 of [16].

when

When the Charging Station detects
that the current Charging Station
certificate will expire in one month.

This requirement is currently hardcoded in OCPP1.6 and OCPP2.0.1:
https://github.com/EVerest/libocpp/blob/main/lib/ocpp/v16/charge_point_impl.cpp#L120
https://github.com/EVerest/libocpp/blob/main/lib/ocpp/v2/functional_blocks/security.cpp#L458

Describe your solution

In case certificates with a very short validity period are installed, it might not be intended to renew the certificate at startup. It could be useful to to have the renewal threshold configurable.

Additional context

Possible configuration key / variable name:
CertificateRenewalThreshold