Allow users to specify MQTT broker keep alive interval and ping frequency

Just a followup thought here: might it be useful to change these in specific circumstances? Like if:
- a user has a particularly flaky network, "i know my wifi drops daily and it takes 2 minutes for the network to reboot"
- a user needs a tighter disconnect period, "5 seconds is make or break, kick my device at 7 seconds without heartbeat!"
...if they could control these values they could self-help wrt disconnect behavior

We could allow these values to be overridden by, for instance:
- user controls in the frontend
- special `secrets.json` vars

_Originally posted by @lorennorman in https://github.com/adafruit/Adafruit_Wippersnapper_Arduino/issues/900#issuecomment-4240135640_

For now we can relatively easily update the keep alive interval, potentially via config/secrets, as it's a full blown property:
https://github.com/adafruit/Adafruit_Wippersnapper_Arduino/blob/c212b86a0340c0552826e8deed36afa0c74a7b82/src/Wippersnapper.h#L481-L482

Start in src/Wippersnapper.h around lines 481-482, where the keep-alive interval is exposed, and trace how configuration or secrets.json values are loaded. Define how users can override the keep-alive interval and ping frequency, then verify that the resulting MQTT disconnect behavior follows those configured values.

Written by the indexing model from the issue text.