Allow passing custom query params to minio

Part of wiring up custom parameters to server access logging. This is
done by adding additional query parameters prefixed with `x-` to
requests.

This contains support for GET requests only and must be used in
conjunction with an updated version of minio.

repo/blob/s3/s3_options.go

@@ -23,6 +23,7 @@ type Options struct {
 	SecretAccessKey string            `json:"secretAccessKey" kopia:"sensitive"`
 	SessionToken    string            `json:"sessionToken" kopia:"sensitive"`
 	Tags            map[string]string `json:"tags" kopia:"sensitive"`
+	QueryParams     map[string]string `json:"queryParams" kopia:"sensitive"`
 	RoleARN         string            `json:"roleARN"`
 	SessionName     string            `json:"sessionName"`
 	RoleDuration    string            `json:"duration"`

repo/blob/s3/s3_storage.go

@@ -48,6 +48,10 @@ func (s *s3Storage) getBlobWithVersion(ctx context.Context, b blob.ID, version s
 	attempt := func() error {
 		opt := minio.GetObjectOptions{VersionID: version}
 
+		for k, v := range s.QueryParams {
+			opt.AddReqParam(k, v)
+		}
+
 		if length > 0 {
 			if err := opt.SetRange(offset, offset+length-1); err != nil {
 				return errors.Wrap(blob.ErrInvalidRange, "unable to set range")