micro: rewrite options to support multiple building blocks

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

context.go

@@ -0,0 +1,22 @@
+package micro
+
+import "context"
+
+type serviceKey struct{}
+
+// FromContext retrieves a Service from the Context.
+func FromContext(ctx context.Context) (Service, bool) {
+	if ctx == nil {
+		return nil, false
+	}
+	s, ok := ctx.Value(serviceKey{}).(Service)
+	return s, ok
+}
+
+// NewContext returns a new Context with the Service embedded within it.
+func NewContext(ctx context.Context, s Service) context.Context {
+	if ctx == nil {
+		ctx = context.Background()
+	}
+	return context.WithValue(ctx, serviceKey{}, s)
+}