Allow DS18X20_MAX_SENSORS to be redefined

[barbudor]

Description:

Following discussion with users on discord, this PR allows DS18X20_MAX_SENSORS to be redefined in user_config_override.h.
If merged I will document with warnings regarding MQTT buffer size and 1-wire length reliability.
Build as been tested with discord user earthquake on ESP8266 only

Checklist:

• The pull request is done against the latest dev branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works on Tasmota core ESP8266 V.2.7.4.9
• The code change is tested and works on Tasmota core ESP32 V.1.0.4.2
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass