MM-8602: KV Store Json helper tweaks (#11502)

* move kv helpers to helpers_kv*.go

* change KVGetJSON return signature

Return a boolean and an error, to clearly indicate if no value was found and thus no value unmarshalled into the target interface.

Also fix an issue with CompareAndSet to allow an oldValue of nil (i.e.
expected to be unset).

* add missing license

* tweak documentation

* document KVSetWithExpiryJSON minimum version

plugin/plugintest/helpers.go

@@ -55,17 +55,24 @@ func (_m *Helpers) KVCompareAndSetJSON(key string, oldValue interface{}, newValu
 }
 
 // KVGetJSON provides a mock function with given fields: key, value
-func (_m *Helpers) KVGetJSON(key string, value interface{}) error {
+func (_m *Helpers) KVGetJSON(key string, value interface{}) (bool, error) {
 	ret := _m.Called(key, value)
 
-	var r0 error
-	if rf, ok := ret.Get(0).(func(string, interface{}) error); ok {
+	var r0 bool
+	if rf, ok := ret.Get(0).(func(string, interface{}) bool); ok {
 		r0 = rf(key, value)
 	} else {
-		r0 = ret.Error(0)
+		r0 = ret.Get(0).(bool)
 	}
 
-	return r0
+	var r1 error
+	if rf, ok := ret.Get(1).(func(string, interface{}) error); ok {
+		r1 = rf(key, value)
+	} else {
+		r1 = ret.Error(1)
+	}
+
+	return r0, r1
 }
 
 // KVSetJSON provides a mock function with given fields: key, value