add selfsigned cert generation api

lib/libwebsockets.h

lib/private-libwebsockets.h

lib/server/ssl-server.c

lib/tls/mbedtls/server.c

lib/tls/mbedtls/ssl.c

lib/tls/openssl/lws-genrsa.c

lib/tls/openssl/server.c

lib/tls/openssl/ssl.c