Added support for chunked uploads. (#1208)

* Added tests for mid-size and big artifacts, reproducing a problem with chunked uploads.

* Added support for chunked uploads.

* Enforced overwriting uploaded artifacts on receiving the first chunk.

Co-authored-by: Casey Lee <cplee@nektos.com>

pkg/artifacts/server_test.go

@@ -51,6 +51,16 @@ func (fsys MapFsImpl) Open(path string) (fs.File, error) {
 	return WritableFile{result, fsys.MapFS, path}, err
 }
 
+func (fsys MapFsImpl) OpenAtEnd(path string) (fs.File, error) {
+	var file = fstest.MapFile{
+		Data: []byte("content2"),
+	}
+	fsys.MapFS[path] = &file
+
+	result, err := fsys.MapFS.Open(path)
+	return WritableFile{result, fsys.MapFS, path}, err
+}
+
 func TestNewArtifactUploadPrepare(t *testing.T) {
 	assert := assert.New(t)