fix(utilities): hold_warnings is public but missing from pcapkit.utilities.warnings.__all__ #1602

pcapkit.utilities.warnings.hold_warnings is public: it is documented by .. autoclass:: in docs/source/pcapkit/utilities/functools.rst:46 and imported by pcapkit/protocols/protocol.py:53. But it is missing from that module's __all__ (pcapkit/utilities/warnings.py:78). That list has 24 entries, warn and all 23 warning classes, measured on main 4ee525e21. As a result, from pcapkit.utilities.warnings import * does not bring it in. The stdlib exports its counterpart warnings.catch_warnings the same way.

Change: add 'hold_warnings' beside 'warn', with a test that fails without it. Per the ruling on #1599 the name stays lowercase.

Found by the #1601 cross-review; I verified the 24 entries by AST.