fix some compiler warnings and PlatformIO config

• fixed compiler warnings (comparison of constant is always true, catching polymorphic type by value)
• fixed platform_packages option
• updated dependencies for GitHub Actions (Arduino Core for ESP8266 3.1.2, ArduinoJson 6.21.3)

updated dependencies

• ESP8266 SDK 3.1.2
• ArduinoJSON 6.21.3

support PlatformIO

• move firmware code and config data into src folder
• fix compiler warnings
• updated README
• updated GitHub build action

MQTT port configuration

optionally specify MQTT server port in config file

improved setting desired water temperature

set desired water temperature without interrupting MQTT connection

set disinfection time for model SJB-HS

changed data type of disinfection time setpoint from bool (on/off) to int (hours)

get disinfection time for model SJB-HS

changed data type of actual disinfection time from bool (on/off) to int (hours)

code restructured to improve support for model SJB-HS

• support more than 16 symbols per 7-segment display
• support reading disinfection time for model SJB-HS
• changed error code data type from uint32 to string to make it human readable

SJB-HS model

support both Intex PureSpa SB-H20 and SJB-HS models, selectable during build using a #define in source file common.h

MQTT authentication

support MQTT authentication with username/password