Add getters for RX1 and RX2 windows

[sebromero]

To minimize the wait time for downlink messages it's necessary to know the delay times of the RX1 and RX2 windows before any data can be available. After a call to poll() the device needs to wait at least until the RX1 window has passed. This PR implements the getter functions to do the math.
That way downlink messages can be retrieved reliably and with minimal wait time. E.g.

modem.poll();
delay(modem.getRX1Delay() + 1000); // Allow for a bit of extra time for the transmission of the data
while (modem.available()) {   
  // Fetch data from the modem
}

[sebromero]

@facchinm The RX1 getter works but for some reason I can't get the RX2 getter to work.

Might be because of the missing AT_PRINTF("+OK="); statement here: https://github.com/arduino/mkrwan1300-fw/blob/b1c48b27280cb455ebe73f723e4a6cc01854d1f5/Projects/Multi/Applications/LoRa/AT_Slave/src/at.c#L672
Then again, a simple check waitResponse() == 1 should work, given that the function returns AT_OK.

Any hint on what's going on? I might be overlooking something.

[github-actions]

Memory usage change @ 2e86755

Board	flash	%	RAM for global variables	%
arduino:mbed_portenta:envie_m4	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:mbed_portenta:envie_m7	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:samd:mkrwan1300	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:samd:mkrwan1310	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0

Click for full report table

Board	examples/DumbModemLoraSender flash	%	examples/DumbModemLoraSender RAM for global variables	%	examples/FirstConfiguration flash	%	examples/FirstConfiguration RAM for global variables	%	examples/LoraSendAndReceive flash	%	examples/LoraSendAndReceive RAM for global variables	%	examples/FWUpdaterBridge flash	%	examples/FWUpdaterBridge RAM for global variables	%	examples/MKRWANFWUpdate_standalone flash	%	examples/MKRWANFWUpdate_standalone RAM for global variables	%
arduino:mbed_portenta:envie_m4	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0
arduino:mbed_portenta:envie_m7	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0
arduino:samd:mkrwan1300	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0
arduino:samd:mkrwan1310	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0

Click for full report CSV

Board,examples/DumbModemLoraSender<br>flash,%,examples/DumbModemLoraSender<br>RAM for global variables,%,examples/FirstConfiguration<br>flash,%,examples/FirstConfiguration<br>RAM for global variables,%,examples/LoraSendAndReceive<br>flash,%,examples/LoraSendAndReceive<br>RAM for global variables,%,examples/FWUpdaterBridge<br>flash,%,examples/FWUpdaterBridge<br>RAM for global variables,%,examples/MKRWANFWUpdate_standalone<br>flash,%,examples/MKRWANFWUpdate_standalone<br>RAM for global variables,%
arduino:mbed_portenta:envie_m4,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0
arduino:mbed_portenta:envie_m7,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0
arduino:samd:mkrwan1300,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0
arduino:samd:mkrwan1310,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0

[flhofer]

@sebromero
Hi, I fixed many of these issues in my firmware pull request. You might need to adapt to the matching library version though. Anyway, the library uses internal setters and getters, making the change mostly one-liners. 😃 (i did it for my tests)
Hope it helps
(EDIT: I chose not to commit them because (a) you use them only in non-standard systems, and (b) it's easy to add in the new version)