Masterlock: Get temporary code

Returns a temporary code for the lock.

Most Masterlocks rotate the codes 6 times per day at fixed intervals.
These intervals put you at risk of receiving a soon-to-expire code. We don't do any magic to mitigate this,
but simply allow for you to supply a validAt query parameter, asking for a code in the near future.

I.e. if you ask for a code valid at 03:59 and it is rotated at 04:00, the user will have 1 minute to input the code.
In these cases you may want to relay the next code as well. Use the first code's validTo property to request the next code.