Slack Slash Command (application/x-www-form-urlencoded) -> API Gateway -> Lambda (application/json)
#set($httpPost = $input.path('$').split("&"))
{
#foreach( $keyValue in $httpPost )
#set($data = $keyValue.split("="))
"$data[0]" : "$data[1]"#if( $foreach.hasNext ),#end
#end
}