feat(lagoon/attach): export UploadWriterOptions for blob writers that bypass Store (quick-261106-d20)

- attach.UploadWriterOptions (was the unexported uploadWriterOptions) gives
  code that writes an upload to the bucket itself the same Content-Type and
  Content-Disposition: attachment for non-inline types that attach.Store stores
- writeBlob calls it; Store behaviour unchanged
- TestUploadWriterOptions table test
- lagoon README, docs/database/attachments.md and docs/services/storage.md
  describe it for writers that bypass attach.Store

modules/lagoon/attach/store.go

@@ -205,7 +205,7 @@ func Store(ctx context.Context, db *gorm.DB, bucket *blob.Bucket, in Upload, lim
 func writeBlob(ctx context.Context, bucket *blob.Bucket, key string, r io.Reader, contentType string, limit int64) (int64, error) {
 	writeCtx, cancel := context.WithCancel(ctx)
 	defer cancel()
-	w, err := bucket.NewWriter(writeCtx, key, uploadWriterOptions(contentType))
+	w, err := bucket.NewWriter(writeCtx, key, UploadWriterOptions(contentType))
 	if err != nil {
 		return 0, fmt.Errorf("attach: blob writer: %w", err)
 	}