feat(rum): support remote configuration in Flutter

[guguji5]

支持配置动态下发

ios 模拟器测试了 远程配置下发，beforeSampling和getRemoteConfig

[guguji5]

小程序端的pr在 flashcatcloud/fc-sdk-wechat-miniprogram#18 ，里边几个方法可以参考里边的说明

[guguji5]

安卓真机上也调试了一下，也有对应的日志输出

[Fiona2016]

[P2] Preserve explicit null values in the beforeSampling context

The default Gson encoder omits object entries whose value is null, including nested entries. For example, custom = {"override": null} reaches the Dart callback as {}, while getRemoteConfig() and the iOS bridge preserve that entry. A sampling callback that uses containsKey to distinguish an absent setting from an explicitly cleared setting can therefore make a different decision on Android.

Please use GsonBuilder().serializeNulls().create() here and add coverage for both top-level and nested null-valued custom entries. Reproduced with Gson 2.12.1: the current encoder drops those entries, while a serializeNulls() control preserves them.