Add compress_threshold protocol option

Currently the compression threshold is set to 300 and hardcoded in the
codebase. There are cases where it make sense to allow this to be
configured, for instance when you want to enforce all responses to be
compressed regarldess of their size.

src/cowboy_http.erl

@@ -25,6 +25,7 @@
 -export([system_code_change/4]).
 
 -type opts() :: #{
+	compress_threshold => non_neg_integer(),
 	connection_type => worker | supervisor,
 	env => cowboy_middleware:env(),
 	idle_timeout => timeout(),
@@ -46,7 +47,9 @@
 	shutdown_timeout => timeout(),
 	stream_handlers => [module()],
 	tracer_callback => cowboy_tracer_h:tracer_callback(),
-	tracer_match_specs => cowboy_tracer_h:tracer_match_specs()
+	tracer_match_specs => cowboy_tracer_h:tracer_match_specs(),
+	%% Open ended because configured stream handlers might add options.
+	_ => _
 }.
 -export_type([opts/0]).