Different baud rates for different modules

Copied from @PaulZC in #1:

We need a way for each derived class to be able to override the supported baud rates. I believe the code is currently defaulting to 115200, which is OK and works on the SARA. But SARA needs to support slower bauds too:

![image](https://github.com/sparkfun/SparkFun_u-blox_AT_Commands_Arduino_Library/assets/102242586/0c5669f6-18aa-4cd1-af78-0c44a1dca590)