add context helpers

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

store/context.go

@@ -0,0 +1,26 @@
+package store
+
+import (
+	"context"
+)
+
+type storeKey struct{}
+
+func storeContext(ctx context.Context) (Store, bool) {
+	c, ok := ctx.Value(storeKey{}).(Store)
+	return c, ok
+}
+
+func NewContext(ctx context.Context, c Store) context.Context {
+	return context.WithValue(ctx, storeKey{}, c)
+}
+
+// SetOption returns a function to setup a context with given value
+func SetOption(k, v interface{}) Option {
+	return func(o *Options) {
+		if o.Context == nil {
+			o.Context = context.Background()
+		}
+		o.Context = context.WithValue(o.Context, k, v)
+	}
+}