Added streaming support to scala-native curl-based sync backend (#2807)

Added sync streaming support to scala-native curl-based sync backend
using curl_multi api family

Before submitting pull request:
- [X] Check if the project compiles by running `sbt compile`
- [X] Verify docs compilation by running `sbt compileDocs`
- [ ] Check if tests pass by running `sbt test`
- [X] Format code by running `sbt scalafmt`

Author: lbialy

core/src/main/resources/scala-native/ffi.c

@@ -8,4 +8,18 @@ const char* sttp_curl_get_version() {
     return curl_version_info(CURLVERSION_NOW)->version;
 }
 int sttp_curl_getinfo_pointer(CURL *curl, CURLINFO info, void* arg) {return curl_easy_getinfo(curl, info, arg); }
+
+/* curl_multi wrappers */
+/* Most curl_multi functions are called directly via @extern bindings.
+   This wrapper exists because CURLMsg contains a union that can't be
+   safely represented in Scala Native's type system. */
+int sttp_curl_multi_info_read_result(CURLM *multi, CURL **easy_out) {
+    int msgs_in_queue;
+    CURLMsg *msg = curl_multi_info_read(multi, &msgs_in_queue);
+    if (msg && msg->msg == CURLMSG_DONE) {
+        if (easy_out) *easy_out = msg->easy_handle;
+        return (int)msg->data.result;
+    }
+    return -1; /* no completed transfer */
+}
 #endif

core/src/main/scalanative/sttp/client4/SttpExtensions.scala

@@ -1,12 +1,22 @@
 package sttp.client4
 
-import java.io.File
+import java.io.{File, InputStream}
 import java.nio.file.Path
 
 import sttp.client4.internal._
 import sttp.model.{Part, StatusCode}
 
 trait SttpExtensions {
+
+  def asInputStream[T](f: InputStream => T): ResponseAs[Either[String, T]] =
+    asEither(asStringAlways, asInputStreamAlways(f))
+
+  def asInputStreamAlways[T](f: InputStream => T): ResponseAs[T] = new ResponseAs(ResponseAsInputStream(f))
+
+  def asInputStreamUnsafe: ResponseAs[Either[String, InputStream]] = asEither(asStringAlways, asInputStreamAlwaysUnsafe)
+
+  def asInputStreamAlwaysUnsafe: ResponseAs[InputStream] = new ResponseAs(ResponseAsInputStreamUnsafe)
+
   def asFile(file: File): ResponseAs[Either[String, File]] = asEither(asStringAlways, asFileAlways(file))
 
   def asFileAlways(file: File): ResponseAs[File] =