Add CallsiteParameter.TASK_NAME to support taskName attribute

[aoright]

This PR adds CallsiteParameter.TASK_NAME which extracts the current asyncio task name. This brings structlog up to parity with standard library logging starting in Python 3.12, where taskName was introduced to LogRecord.

Closes #693.

[codspeed]

Merging this PR will not alter performance

✅ 10 untouched benchmarks

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_{Comparing aoright:fix-task-name (d7016a9) with main (bf3cfd0)¹}

Footnotes

1. No successful run was found on main (73393f3) during the generation of this report, so bf3cfd0 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩