Add function to reload WS creds from configs

[matth-x]

The Mongoose WS adapter needs to react differently when the OCPP server updates the WS credentials (e.g. "Cst_BackendUrl") vs. the client code locally updates the WS credentials. This PR changes the API to reduce the complexity of the differentiation:

• void setBackendUrl(const char*), void setChargeBoxId(const char*) and void setAuthKey(const char*) don't take immediate effect anymore. The credentials updates need to be applied manually using void reloadConfigs()
• the client code can also update the WS credentials by programmatically updating the underlying OCPP configs and apply the updates using void reloadConfigs()
• void reconnect() is not part of the configs update procedure anymore and was changed to private