unistore: move continue to the resource package (#101206)

* minor improvements to tests

* move continue token

* update sql backend

pkg/storage/unified/resource/continue_test.go

@@ -0,0 +1,15 @@
+package resource
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestContinueToken(t *testing.T) {
+	token := &ContinueToken{
+		ResourceVersion: 100,
+		StartOffset:     50,
+	}
+	assert.Equal(t, "eyJvIjo1MCwidiI6MTAwfQ==", token.String())
+}