[FEATURE]: iLogtail support prometheus Flusher [flusher_prometheus]

[aiops1900]

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新增一个flusher插件[flusher_prometheus]，用于将采集数据上报到支持Prometheus协议RemoteWrite的RemoteStorage（e.g. Prometheus, VictoriaMetrics, etc.）。

Describe the solution you'd like (if any)
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它的实现，需要依赖/组合多个其它插件（包括但不限于）：
flusher_http [可以考虑作为flusher_prometheus的父类，并需要对flusher_http做一些增强，比如支持ext_default_encoder集成 #1662 ]
ext_default_encoder [比如基于增强的flusher_http]
prometheus encode实现 [ext_default_encoder的配置中指定 prometheus encode 实现]
[选]ext_basicauth [也可以直接用HTTP Header的Authentication字段来完成BasicAuth鉴权信息注入]
...

Additional comments
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理想的配置用法：

flushers:
- Type: flusher_prometheus
  Endpoint: 'http://xxxx/api/v1/write'
  Authenticator:
    Type: ext_basicauth
extensions:
- Type: ext_basicauth
  Username: '$YOUR_USENAME'
  Password: '$YOUR_PASSWORD'

说明：考虑到pipeline配置的灵活性，且extension支持由pipeline全局初始化和插件局部初始化2种，所以上面只是其中一种配置示例。具体实现也取决于最终方案。