External I2S pin configuration (AUD-1392)

[apiesse]

I2S MCLK can be configured on any pin using ledc peripheral (PWM timers) just like esp32-camera does.
I2S pin can be passed on the i2s_stream object: having I2S pins hardcoded into the board get_i2s_pins() funcion can be limiting if you want to support different HW configurations without the need of different binaries (buildling the same fiware with different board enabled).

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