New Feature: Support Adafruit STEMMA Seesaw capacitive soil moisture sensor (i2c)

[vic42]

Description:

This PR adds support for up to 4 Adafruit STEMMA (Seesaw) capacitive soil moisture sensors to be connected to I2C. It is based on the Adafruit_seesaw_soilsensor_1.3.1 library which is part of this PR.
By default the driver emits MQTT updates every TelePeriod only. There is an option to #define SEESAW_SOIL_PUBLISH to emit MQTT updates immediately whenever the moisture value changes.
I chose not to include it by default because I won't usually expect soil moisture changes need to be acted upon immediately. Product description: https://www.adafruit.com/product/4026

Related issue (if applicable): "fixes" #8467

Checklist:

• The pull request is done against the latest development 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.1.0.5-rc4
• I accept the CLA.

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