MEDIUM: applet: Simplify a bit API to exchange data with applets

Default .rcv_buf and .snd_buf functions that applets can use are now
specialized to manipulate raw buffers or HTX buffers.

Thus a TCP applet should use appctx_raw_rcv_buf() and appctx_raw_snd_buf()
while HTTP applet should use appctx_htx_rcv_buf() and appctx_htx_snd_buf().

Note that the appctx is now directly passed to these functions instead of
the SC.

src/cache.c

@@ -3159,8 +3159,8 @@ struct applet http_cache_applet = {
 	.obj_type = OBJ_TYPE_APPLET,
 	.name = "<CACHE>", /* used for logging */
 	.fct = http_cache_io_handler,
-	.rcv_buf = appctx_rcv_buf,
-	.snd_buf = appctx_snd_buf,
+	.rcv_buf = appctx_htx_rcv_buf,
+	.snd_buf = appctx_htx_snd_buf,
 	.fastfwd = http_cache_fastfwd,
 	.release = http_cache_applet_release,
 };