[FEATURE]: iLogtail的extension新增encoder接口

[aiops1900]

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iLogtail的 pkg/pipeline/extensions 中，已经提供了 ClientAuthenticator、Decoder 等 extension 接口，这里需要新增一个 Encoder 接口，用于各类开源协议encode driver的实现依赖（e.g. Prometheus、Influxdb等）

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尽管现阶段各类开源协议的encode实现，都在pkg/protocol/converter中，但从软件设计看，实现不是很优雅，可以参照Decoder的思路，将encode实现也用Encoder接口做抽象统一，结合Pipeline Config灵活选择使用

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For example, the ideal input and output logs.

接口形式（参考Decoder，区分SLS Log和PipelineGroupEvents模型，分别对应EncoderV1和EncoderV2）

// Encoder encodes data of iLogtail data models into bytes.
// Different drivers with different encoding protocols implement Encoder interface.
//
// drivers: raw, influxdb, prometheus, sls, ...
type Encoder interface {
	EncoderV1
	EncoderV2
}

// EncoderV1 supports v1 pipeline plugin interface,
// encodes data of v1 model into bytes.
//
// drivers: sls, influxdb, ...
type EncoderV1 interface {
	EncodeV1(*protocol.LogGroup) ([][]byte, error)
	EncodeBatchV1([]*protocol.LogGroup) ([][]byte, error)
}

// EncoderV2 supports v2 pipeline plugin interface,
// encodes data of v2 model into bytes.
//
// drivers: raw, influxdb, prometheus, ...
type EncoderV2 interface {
	EncodeV2(*models.PipelineGroupEvents) ([][]byte, error)
	EncodeBatchV2([]*models.PipelineGroupEvents) ([][]byte, error)
}

type EncoderExtension interface {
	Encoder
	pipeline.Extension
}