sort keys in Settings.Keys() method for consistent order (fix manifest reconcile issue)

[mastercactapus]

This pull request introduces a minor improvement to the Keys method in the Settings struct by ensuring the returned list of keys is sorted alphabetically.

This is to correct an issue we found where pods are re-created by the stateful set for (possibly) every reconcile due to the generated volume mounts getting re-ordered (e.g., using a secret for TLS key/cert as documented here) and possibly others.

The value of .Keys() is used in a few different places; notably by WalkKeysSafe, used by WalkSafe, and finally normalizeConfigurationFiles. Since Go map iterations are random otherwise, sorting them here means the WalkSafe method will "walk" in alphabetical order, making the generated mounts/volume order stable.

// normalizeConfigurationFiles normalizes .spec.configuration.files
func (n *Normalizer) normalizeConfigurationFiles(files *chi.Settings, scope any) *chi.Settings {
	if files == nil {
		return nil
	}
	files.Normalize(n.settingsNormalizerOptions(replacerFiles, scope))

	files.WalkSafe(func(key string, setting *chi.Setting) {
		subst.ReplaceSettingsFieldWithMountedFile(n.req, files, key)
	})

	return files
}

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