testing conditions:
transports (tcp, ssl, ws..)
qos (0, 1, 2)
short repeated messages (packed packets)
oversized messages (fragmented packets)
publish from a different thread
Closes https://github.com/espressif/esp-idf/issues/2870 by means of including commit 815623dfe5a0e41fa0e51ab4e336feb3eaa5ba15 from esp-mqtt
Closes https://github.com/espressif/esp-idf/issues/2975 by means of including commit 752953dc3be007cca4255b66a35d3087e61f6a54 from esp-mqtt
Closes https://github.com/espressif/esp-idf/issues/2850 by means of including commits df455d2a5fe562dd1b8351da99a1d6d82b66eff3 17fd713bced4f2d00df7ed664ed82a7d108ab317 from esp-mqtt
added docs and tests for mqtt library, small fixes (removed warnings, option for custom outbox, websocket bug fixed for longer transports). refactored to use common tcp_transport component, support for CMake build system.
Closes#2108