MiniMQTT is losing legacy set_socket method

[justmobilize]

Hello!

I am working through a project to simplify socket handling and unifying methods. This includes the removal of the set_socket methods that were originally designed around the wiznet5k and esp32spi.

This library calls set_socket in 2 places, which implies it supports the esp32spi, but in reality doesn't, since it also requires SSL.

I'm happy to update this library as it will soon become unusable and see three options:

1. Fully remove esp32spi support. This is the easiest, since it's really just removing the legacy method, changing it to store the socketpool internally, and removing old docs and examples
2. Add support for esp32spi, more work and testing, but not complicated as I've done this work elsewhere.
3. Leave it be

Please let me know if I can help keep this library alive.

[justmobilize]

Did 1.1 - added a note about ESP32SPI not currently working. PR here: #63