[ENHANCEMENT]: iLogtail的flusher_http支持extension Encoder且适配相应的逻辑

[aiops1900]

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iLogtail的flusher_http插件，支持extension Encoder（通过*extensions.ExtensionConfig类型的字段配置注入）

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适配Encoder的逻辑
需要考虑兼容之前的Converter的配置[优先级低于Converter配置]

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配置参考 #1662 中的 flusher_http 的配置示例：

flusher:
- Type: flusher_http
  ...
  Encoder:
    Type: ext_default_encoder
    Options:
      Format: 'prometheus'
      ...