[FEATURE]: iLogtail的extension插件新增encoder extension插件[ext_default_encoder]

[aiops1900]

Concisely describe the proposed feature A clear and concise description of what you want. For example,

各采集协议encode实现策略配置的总入口，效果类似ext_default_decoder插件的作用。

Describe the solution you'd like (if any) A clear and concise description of what you want to achieve and implement. For example,

实现参考ext_default_decoder。一般和各类flusher插件组合使用，通过配置的方式选择具体的encode协议。

Additional comments Add any other context or screenshots about the feature request here. For example, the ideal input and output logs.

以flusher_http的配置为例，集成Prometheus Encoder的配置示例

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

说明：这里只是举例用法，不是最终实现方案