I2S: multiple fixes

[Staars]

Description:

• better handle interruptions of operations, i.e. launching i2splay /file.mp3 while playing web radio
• reducing noise before and after playing operations by adding an internal flush and preload method
• more granular setting of volume (4 times the resolution) as my test with a PCM5102 via headphone was too rough with the old version
• allow playing of mono MP3, which produced a ton of noise on the unused channel before in my tests, by modifying the framework
• now correctly showing configured input and output on SOC's with 2 i2s channels
• use correct output gain from settings file after boot
• reject wrong sample rates for the MP3 encoder
• a bit more logging as more issues are to be expected

Tested on a S3 dev board with a cheap PCM5102 and different connected microphones (PDM and INMP441), which interestingly absolutely needed the microphone input on the first channel (and the output on the second).
More research needed ...

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 with Tasmota core ESP8266 V.2.7.6
• The code change is tested and works with Tasmota core ESP32 V.3.0.0
• I accept the CLA.

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