credential: fix lint errors (#3746)

The existing pattern was subject to a bug whereby a lock
value was copied every time the value was passed around.
This was happening because of the use of an embedded
protobuf type. To avoid copying a mutex, use a pointer
type in the embedding.

Also fixes a few linter warnings complaining about the use
of unkeyed literals.

internal/daemon/controller/handlers/credentials/credential_service.go

@@ -718,7 +718,7 @@ func toJsonStorageCredential(ctx context.Context, storeId string, in *pb.Credent
 		ctx,
 		storeId,
 		credential.JsonObject{
-			*object,
+			Struct: object,
 		},
 		opts...)
 	if err != nil {

internal/daemon/controller/handlers/credentials/credential_service_test.go

@@ -727,7 +727,7 @@ func TestCreate(t *testing.T) {
 				Type:              credential.JsonSubtype.String(),
 				Attrs: &pb.Credential_JsonAttributes{
 					JsonAttributes: &pb.JsonAttributes{
-						Object: &obj.Struct,
+						Object: obj.Struct,
 					},
 				},
 			}},