Add retention config for snbk

[jameslai-dev]

This PR adds retention-policy and retention-config entries to the snbk BackupConfig.
The retention-policy supports default and custom options. The default policy is designed to keep the snapshots on the backup device identical to those on the source device (the existing behavior); the custom policy requires an additional configuration entry retention-config, which defines the cleanup behavior.

The parameters under the retention-config section share the same names as snapper configuration options (NUMBER_LIMIT, TIMELINE_LIMIT_HOURLY, ...), so that the cleanup parameter class (the Parameters class in client/cleanup.cc) can load the config without a major interface modification.

An example config:

{
  "config": "test",
  "target-mode": "local",
  "target-path": "/mnt/test-btrfs-target/.snapper/data",
  "automatic": false,
  "retention-policy": "custom",
  "retention-config": {
    "NUMBER_MIN_AGE": "3600",
    "NUMBER_LIMIT": "50",
    "NUMBER_LIMIT_IMPORTANT": "10",
    "TIMELINE_MIN_AGE": "3600",
    "TIMELINE_LIMIT_HOURLY": "48",
    "TIMELINE_LIMIT_DAILY": "14",
    "TIMELINE_LIMIT_WEEKLY": "12",
  }
}

Additional changes:

• A specialized get_child_value function is added to the JsonFile module to support parsing a JSON node into a string-to-string dictionary.

Things to discuss:

• The style of cleanup parameters (NUMBER_LIMIT, TIMELINE_LIMIT_HOURLY, ...) does not match the existing snbk configuration style (target-mode, target-path, ...). It's fine with me, but I'm not sure what others think.

Parent issue: #1105