Expose dissector_add_for_decode_as() to Lua

Allow Lua scripts to add their Protocol's dissector for "Decode as...".

Bug: 10696
Change-Id: Ic270cc85eff62ccfc29d8e5fcbb48247cfcd14bd
Reviewed-on: https://code.wireshark.org/review/6050
Petri-Dish: Hadriel Kaplan <hadrielk@yahoo.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>