feat: Add SNR calculation functionality (Int & Float)

[ArshiaMadadii]

Functionality Implemented

This PR adds support for reading the Signal-to-Noise Ratio (SNR) of the last received packet, which is critical for evaluating link quality in LoRa communications.

The implementation reads from the RegPktSnrValue (0x19) register. Since the SX127x module reports SNR in 0.25dB steps using a signed 2's complement format, I implemented logic to correctly handle negative values (common in long-range transmission).

Outputs

I have added two functions to offer flexibility between precision and performance:

1. LoRa_getSNR_x4(LoRa* _LoRa)

   • Type: int16_t
   • Description: Returns the SNR value in "Quarter-dB" units. This avoids floating-point arithmetic.
   • Example Output: A return value of -50 corresponds to -12.5 dB.

2. LoRa_getSNR(LoRa* _LoRa)

   • Type: float
   • Description: Returns the standard SNR value in dB.
   • Example Output: -12.5