Add connected callback on http.c module

Our project has been using the libevent library, and recently the project has a requirement to add support for HTTP2. The initial scheme reuses the http module in libevent to implement the HTTP2 protocol (the nghttp2 library is additionally used). The nghttp2 library needs to do some initialization after the https handshake is successful, but we checked the http module source code in the libevent library. There is no such callback If you can add this interface, this will be very useful: 1. You can reuse the http1.0 code in the http module, 2. Based on the first point, http1 & http2 share the same http module, which makes development more convenient , I hope to accept it, thank you.
The specific approach is
in evhttp_error_cb add (*http->conncb)(evcon) on what == BEV_EVENT_CONNECTED condition

Start in the http.c implementation at evhttp_error_cb and inspect the BEV_EVENT_CONNECTED branch and existing connection callback handling. Determine how a connected callback can be exposed for the HTTP module, then verify that it is invoked after a successful HTTPS handshake without changing existing HTTP behavior.