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

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.