Optimize JsonSerializer and Flusher_file

[Takuka0311]

优化了JsonSerializer和Flusher_file的性能，主要改动：

• json库替换为rapidjson
• JsonSerializer内使用不释放的buffer优化cpu开销（cpu优化最多）
• 移除Flusher_file内不必要的Batcher
• Flusher_file内spdlog从与loongcollector日志模块共享线程变为独立线程（mem优化最多）

测试场景：10M/s正则解析场景，配置和数据样例列在最后，结果见CI。性能对比如下图。

CPU从平均1.43核降低至0.24核，减少83%：

内存从平均136M降低至30M，减少78%：

附：

配置：

enable: true
inputs:
  - Type: input_file
    FilePaths: 
      - /home/loongcollector/*.log
processors:
  - Type: processor_parse_regex_native
    SourceKey: content
    Regex: ^([^ ]*) ([^ ]*) ([^ ]*) \[([^\]]*)\] "(\S+) ([^\"]*) (\S*)" ([^ ]*) ([^ ]*) "([^\"]*)" "([^\"]*)"
    Keys:
      - ip
      - ident
      - auth
      - timestamp
      - method
      - request
      - http_version
      - response_code
      - bytes
      - referrer
      - user_agent
flushers:
  - Type: flusher_file
    FilePath: /home/loongcollector/test.out

[Copilot]

Pull Request Overview

This PR optimizes performance by replacing the JSON library with rapidjson and refactoring file flushing. The key changes include:

• Replacing the json library with rapidjson in JsonSerializer for faster serialization.
• Removing unused overloads and the Batcher from FlusherFile to reduce CPU overhead.
• Adjusting spdlog configuration to use an independent thread for logging.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
core/plugin/flusher/file/FlusherFile.h	Removed unused SerializeAndPush overload and obsolete variables.
core/plugin/flusher/file/FlusherFile.cpp	Refactored flush logic and updated spdlog thread pool creation; potential misassignment in config parameter.
core/collection_pipeline/serializer/JsonSerializer.cpp	Migrated serialization to rapidjson with helper functions for common fields.