Add support for partial websocket frame payload reads (IDFGH-14913)

[mrsobakin]

Description

Added support for partial websocket frame payload reads. This was implemented as a separate function httpd_ws_recv_frame_part to keep the backwards compatibility with subtle httpd_ws_recv_frame behavior, that being that it returns an undocumented error when max_len < pkt->len.

httpd_ws_frame_t now also contains new left_len field, which is used to keep track of the yet to be received data and calculating the mask offset. In the future, it shall also be used for implementing partial frame sends.

Related

Testing

This feature was tested by me on my project and on some additional edge cases, including trying to over-receive (reading more data than there is in a frame payload) and to under-receive (leaving data unprocessed and unread and completing the request). No unexpected errors were identified.

The change itself is not very complex, however in my opinion it is imperative that someone with more knowledge about the ESP HTTP stack verifies that there is no deeper issues with this implementation (and the idea itself, for that matter), which I may have missed.

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