perf python: Port futex-contention to perf module

Port tools/perf/scripts/python/futex-contention.py to a standalone
script in tools/perf/python/ using the perf module. Avoiding the
embedded interpreter overhead improves execution speed by ~3.2x:

```
$ perf record -e syscalls:sys_*_futex -a sleep 1
...
$ time perf script tools/perf/scripts/python/futex-contention.py
...
real    0m1.007s
user    0m0.935s
sys     0m0.072s
$ time python3 tools/perf/python/futex-contention.py
...
real    0m0.314s
user    0m0.259s
sys     0m0.056s
```

Additional improvements compared to the legacy script:
- Consolidate per-(tid, uaddr) contention count, total_time, min_time,
  and max_time into a single LockStats class instead of maintaining
  three separate dictionaries.
- Validate that uaddr and op tracepoint attributes are present on
  syscalls:sys_enter_futex samples rather than silently attributing
  missing fields to uaddr=0, op=0 (FUTEX_WAIT).
- Add -i/--input CLI option support via argparse and full type
  annotations.

Add a shell test (test_futex_contention_python.sh) to verify the
standalone script.

Assisted-by: Antigravity:gemini-3.1-pro
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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