Migrate stats.py to request helpers

[PaperMtn]

Summary

Migrate secops.chronicle.stats to use request helpers

• Request functions in module now implementing helpers in secops.chronicle.utils.request_utils
  • No as_list option for this module, as it is not relevant
• Update tests in test_stats.py to only test module logic and not centralised error handling, which is already tested
  • Remove redundant test from test_client.py