buildflags: marshal attestations into json with extra attributes correctly

`MarshalJSON` would not include the extra attributes because it iterated
over the target map rather than the source map.

Also fixes JSON unmarshaling for SSH and secrets. The intention was to
unmarshal into the struct, but `UnmarshalText` takes priority over the
default struct unmarshaling so it didn't work as intended.

Tests have been added for all marshaling and unmarshaling methods.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

util/buildflags/secrets.go

@@ -1,6 +1,7 @@
 package buildflags
 
 import (
+	"encoding/json"
 	"strings"
 
 	controllerapi "github.com/docker/buildx/controller/pb"
@@ -73,6 +74,22 @@ func (s *Secret) ToPB() *controllerapi.Secret {
 	}
 }
 
+func (s *Secret) UnmarshalJSON(data []byte) error {
+	var v struct {
+		ID       string `json:"id,omitempty"`
+		FilePath string `json:"src,omitempty"`
+		Env      string `json:"env,omitempty"`
+	}
+	if err := json.Unmarshal(data, &v); err != nil {
+		return err
+	}
+
+	s.ID = v.ID
+	s.FilePath = v.FilePath
+	s.Env = v.Env
+	return nil
+}
+
 func (s *Secret) UnmarshalText(text []byte) error {
 	value := string(text)
 	fields, err := csvvalue.Fields(value, nil)