Add JSON output option for show command

[kasunch]

Summary
The show command currently only outputs human-readable formatted text. Add a --json flag to output the effective configuration as structured JSON, making it easier to consume programmatically (e.g., in scripts, CI pipelines, or other tools).

Usage

resticprofile -n myprofile show --json
resticprofile myprofile.show --json

Expected output

  {
    "global": {
      "default-command": "snapshots",
      "initialize": "false",
      ...
    },
    "profile": {
      "repository": "/path/to/repo",
      "password-file": "key",
      ...
    },
    "schedules": [
      {
        "command": "backup",
        "at": ["daily"],
        ...
      }
    ]
  }

• Output uses the same field names as the configuration file (from mapstructure tags).
• Confidential values are masked, same as the text display.
• Zero/empty values and internal fields are omitted, consistent with text output.
• Nested maps and lists are preserved as proper JSON structures.
• The top-level key is "profile" or "group" depending on the target.
• "schedules" is only present when schedules are configured.

[kasunch]

Force-pushed to include a fix for nested maps and lists in the JSON output. Previously, nested structures (maps within maps, mixed-type lists) were flattened to strings like "key1:val1,key2:{a,b}". Now they are preserved as proper JSON objects and arrays.

No changes to the public API or command usage — just corrected the internal serialization in collectLeaf and collectSliceValue.