[build] split out DefaultFileSource and dependents to a separate target

We don't want to link it into the node bindings, so keep it in a separate target

platform/default/file_source_request.cpp

@@ -0,0 +1,37 @@
+#include <mbgl/storage/file_source_request.hpp>
+
+#include <mbgl/actor/mailbox.hpp>
+#include <mbgl/actor/scheduler.hpp>
+
+namespace mbgl {
+
+FileSourceRequest::FileSourceRequest(FileSource::Callback&& callback)
+    : responseCallback(callback)
+    , mailbox(std::make_shared<Mailbox>(*Scheduler::GetCurrent())) {
+}
+
+FileSourceRequest::~FileSourceRequest() {
+    if (cancelCallback) {
+        cancelCallback();
+    }
+
+    mailbox->close();
+}
+
+void FileSourceRequest::onCancel(std::function<void()>&& callback) {
+    cancelCallback = std::move(callback);
+}
+
+void FileSourceRequest::setResponse(const Response& response) {
+    // Copy, because calling the callback will sometimes self
+    // destroy this object. We cannot move because this method
+    // can be called more than one.
+    auto callback = responseCallback;
+    callback(response);
+}
+
+ActorRef<FileSourceRequest> FileSourceRequest::actor() {
+    return ActorRef<FileSourceRequest>(*this, mailbox);
+}
+
+} // namespace mbgl