File storages

By default we support only file and s3 storage. Later on we will provide storages for another providers, etc.
It could be used like here:

if storage == nil {  storage = NewFsStorage()}ret := make([]string, 0)var filename string// files from POST requestfor _, file := range files {  f, err := file.Open()  if err != nil {    return err  }  bytecontent := make([]byte, file.Size)  _, err = f.Read(bytecontent)  if err != nil {    return err  }  filename, err = storage.Save(&FileForStorage{    Content:           bytecontent,    PatternForTheFile: "*." + strings.Split(file.Filename, ".")[1],    Filename:          file.Filename,  })  if err != nil {    return err  }  err = f.Close()  if err != nil {    return err  }  ret = append(ret, filename)}

Or you can use S3 storage provider for your field in the form, like below:

form := core.NewFormFromModelFromGinContext(ctx, modelI, make([]string, 0), fields, true, "", true)iconField, _ := form.FieldRegistry.GetByName("Icon")s3Storage := core.NewAWSS3Storage("test", &core.AWSConfig{  S3: &core.AWSS3Config{    Region:    "{YOUR_REGION}",    AccessKey: "{YOUR_ACCESS_KEY}",    SecretKey: "{YOUR_SECRET_KEY}",  },})s3Storage.(*core.AWSS3Storage).Bucket = "test"s3Storage.(*core.AWSS3Storage).Domain = "https://{YOUR_PART}.s3.eu-central-1.amazonaws.com/test"iconField.FieldConfig.Widget.(*core.FileWidget).Storage = s3Storage