feat: add support for extra_files configuration in ethereum-package

[mattevans]

Depends on: ethpandaops/ethereum-package#1144 (tests will fail until then). Now merged.

---

This PR adds support for the extra_files feature, allowing users to programmatically define file contents that will be mounted into containers at runtime.

Example

  // Create helper for convenient file management
  helper := config.NewExtraFilesHelper()

  // Add inline configuration
  helper.AddFile("beacon-config.yaml", `
  metrics:
    enabled: true
    port: 8080
  `)

  // Add from filesystem
  helper.AddFileFromPath("validator.yaml", "./configs/validator.yaml")

  // Add JSON configuration
  helper.AddJSON("features.json", map[string]interface{}{
      "metrics": true,
      "tracing": false,
  })

  // Create participant that references these files
  participant := config.NewParticipantBuilder().
      WithEL(client.Geth).
      WithCL(client.Lighthouse).
      WithCLExtraMounts(map[string]string{
          "/configs/beacon.yaml": "beacon-config.yaml",  // References extra_files entry
          "/configs/features.json": "features.json",
      }).
      Build()

  // Build configuration with extra files
  networkConfig, err := config.NewConfigBuilder().
      WithParticipant(participant).
      WithExtraFiles(helper.Build()).  // Extra files at root level
      Build()

Generated YAML

  extra_files:
    beacon-config.yaml: |
      metrics:
        enabled: true
        port: 8080
    features.json: |
      {"metrics": true, "tracing": false}

  participants:
    - el_type: geth
      cl_type: lighthouse
      cl_extra_mounts:
        /configs/beacon.yaml: beacon-config.yaml
        /configs/features.json: features.json