Add gunzip middleware to support gzipped payloads

An experiment in adding a simple middleware that will handle gzipped
requests in the collector and process them as usual next.

core/src/main/scala/com.snowplowanalytics.snowplow.collector.core/GUnzipMiddleware.scala

@@ -0,0 +1,41 @@
+/**
+  * Copyright (c) 2013-present Snowplow Analytics Ltd.
+  * All rights reserved.
+  *
+  * This program is licensed to you under the Snowplow Community License Version 1.0,
+  * and you may not use this file except in compliance with the Snowplow Community License Version 1.0.
+  * You may obtain a copy of the Snowplow Community License Version 1.0 at https://docs.snowplow.io/community-license-1.0
+  */
+package com.snowplowanalytics.snowplow.collector.core
+
+import fs2.compression.Compression
+import cats.data.Kleisli
+import cats.effect._
+import org.http4s._
+import org.http4s.headers.`Content-Encoding`
+
+object GUnzipMiddleware {
+  def apply[F[_]: Sync](service: HttpRoutes[F], bufferSize: Int = 100 * 1024): HttpRoutes[F] =
+    Kleisli { (req: Request[F]) =>
+      val req2 = req.headers.get[`Content-Encoding`] match {
+        case Some(header) if satisfiedByGzip(header) =>
+          val decoded = req.body.through(Compression.forSync[F].gunzip(bufferSize)).flatMap(_.content).handleErrorWith {
+            e =>
+              throw MalformedMessageBodyFailure(
+                "Failed to decode gzippped request body",
+                Some(e)
+              )
+          }
+          req
+            .removeHeader[`Content-Encoding`]
+            .withEntity(decoded)(
+              EntityEncoder.entityBodyEncoder
+            ) // resolving implicit conflict
+        case _ => req
+      }
+      service(req2)
+    }
+
+  private def satisfiedByGzip(header: `Content-Encoding`) =
+    header.contentCoding.matches(ContentCoding.gzip) || header.contentCoding.matches(ContentCoding.`x-gzip`)
+}

core/src/main/scala/com.snowplowanalytics.snowplow.collector.core/Routes.scala

@@ -94,7 +94,7 @@ class Routes[F[_]: Sync](
 
   val value: HttpApp[F] = {
     val routes = healthRoutes <+> corsRoute <+> cookieRoutes <+> rootRoute <+> crossdomainRoute
-    val res    = if (enableDefaultRedirect) routes else rejectRedirect <+> routes
+    val res    = if (enableDefaultRedirect) GUnzipMiddleware(routes) else rejectRedirect <+> routes
     res.orNotFound
   }
 }