OCPP: Support uploading root certificates.

[rtrbt]

https://docs.espressif.com/projects/esp-idf/en/release-v4.4/esp32/api-reference/protocols/esp_websocket_client.html#tls

https://www.tinkerunity.org/topic/8025-anschluss-von-warp-charger-pro-1-an-citywatt-abrechnungssystem/?do=findComment&comment=51764

There are other features that will allow using TLS connections in the future (#25 #76). Maybe add a general certificate management sub-page?

[rtrbt]

Done.