【需求】tidevice crashreport只能导出目录，不能导出指定文件

`def cmd_crashreport(args: argparse.Namespace):
d = _udid2device(args.udid)
cm = d.get_crashmanager()
if args.list:
cm.preview()
return
if args.clear:
cm.remove_all()
return
if not args.output_directory:
print("OUTPUT_DIRECTORY must be provided")
sys.exit(1)

remove: bool = not args.keep
cm.afc.pull("/", args.output_directory, remove=remove)
logger.info("Done")`

希望增加对导出单个指定文件的支持

Start at cmd_crashreport and inspect how its command-line arguments are defined, then read the cm.afc.pull call shown in the issue to understand its supported source and destination forms. Done means the crashreport command can export one specified file while preserving directory export, with the relevant command behavior covered by tests if this area has them.

Written by the indexing model from the issue text.