Add a K:V pair to getscinfo that shows the number of blocks of the certificate submission window

[cronicc]

Exposing the length of the certificate submission window (currently 20% of epoch length) in getscinfo would be beneficial. It's possible to calculate it manually, but people might chose to do this in an unsafe way, for instance "ceasing height" - "end epoch height" would not work in case we are in the submission window and the SC has already advanced to the next epoch.

I'd propose "certSubmissionWindowLength": 20, similar to "withdrawalEpochLength": 100,