Add compression method and volume splitting support for 7z compressor

[Copilot]

Adds configuration parameters for 7z to specify compression algorithm and native volume splitting. When using 7z's native -v splitting, the external split_with stage cannot be used simultaneously.

Changes

• method: Compression algorithm selection (-m0=), e.g., LZMA, LZMA2, PPMd, BZip2, Deflate, Copy
• volume_size: Native 7z volume splitting (-v), e.g., "100m", "1g"
• Conflict validation: Returns error if both volume_size and split_with are configured

Example

models:
  my_backup:
    compress_with:
      type: 7z
      method: LZMA2
      compression_level: 9
      volume_size: 100m
    # split_with: ... # Cannot use with volume_size

Original prompt

7z增加配置参数实现支持指定压缩算法，指定分包大小（当使用7z压缩时的原生分包参数，则无法使用原本的Splitter作为下一阶段，如果同时启用，请报错提示）

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[LazuliKao]

@copilot 添加docs