add branch service in core

core/branch/branch.go

@@ -0,0 +1,17 @@
+// Package branch contains the core branch service interface and implementation.
+package branch
+
+import "context"
+
+// Service is the branch service interface.
+type Service interface {
+	// Execute executes the given function in an isolated context.
+	// If the function returns an error, the execution is considered failed,
+	// and every ste change made during the execution is rolled back.
+	// If the function returns nil, the execution is considered successful, and
+	// committed.
+	// The context.Context passed to the `f` function is a child of the context
+	// passed to the Execute function, and is what should be used with other
+	// core services in order to ensure the execution remains isolated.
+	Execute(ctx context.Context, f func(ctx context.Context) error) error
+}