[ENHANCEMENT]: Support json encoding when converter protocol is raw

[xuzhijvn]

Concisely describe the proposed feature

Support json encoding when converter protocol is raw

Describe the solution you'd like (if any)

Currently, converter does not support the case where the protocol is raw and the encoding is json.

For example, service_http_server receives data, processor_custom processes it, and flusher_http sends it out:

enable: true
global:
  StructureType: v2
inputs:
  - Type: service_http_server
    Format: "raw"
    Address: "http://0.0.0.0:12345"
processors:
  - Type: processor_custom
    URL: "https://postman-echo.com/post"
flushers:
  - Type: flusher_stdout
    OnlyStdout: true
  - Type: flusher_http
    RemoteURL: "http://localhost:3000/api/public/ingestion"
    Convert:
      Protocol: raw
      Encoding: json 
    Concurrency: 10
    QueueCapacity: 4096
    DropEventWhenQueueFull: true