Four internal APIs `esp_http_client_connect()`, `http_client_prepare_first_line()`, `esp_http_client_request_send()` and
`esp_http_client_send_post_data()` API have been added for perfoming connection establishment (sync as well as async),
preparing first line, sending request and sending post data respectively.
In `transport_ssl`, `ssl_connect_async()` API has been added for establishing asynchronous connection.
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
