Performance: lib_performance: add numeric helpers for tiotest parsing

Move common numeric/normalization helpers into lib_performance.sh.

- Add norm_metric(), is_number(), append_if_number()
- Keep helpers POSIX/ShellCheck clean and reusable across runners
- No behavior change in existing callers until follow-up commits

Signed-off-by: Srikanth Muppandam <smuppand@qti.qualcomm.com>

Author: smuppand