[MM-20353] Change KVSetWithOptions to accept a byte slice (#13213)

* Drop EncodeJSON from PluginKVSetOptions

* Add unit tests for KVSetWithOptions with nil values

app/plugin_api.go

@@ -697,7 +697,7 @@ func (api *PluginAPI) InstallPlugin(file io.Reader, replace bool) (*model.Manife
 
 // KV Store Section
 
-func (api *PluginAPI) KVSetWithOptions(key string, value interface{}, options model.PluginKVSetOptions) (bool, *model.AppError) {
+func (api *PluginAPI) KVSetWithOptions(key string, value []byte, options model.PluginKVSetOptions) (bool, *model.AppError) {
 	return api.app.SetPluginKeyWithOptions(api.id, key, value, options)
 }