DNM: add tokio taskdump support

[problame]

Usage

Launch with a single runtime (otherwise the task dump only includes those from the mgmt request handle runtime)

NEON_PAGESERVER_USE_ONE_RUNTIME=current_thread ...your_reproducer...

When it's hanging, run

curl $pageserver:$mgmt_api_port/debug/taskdump | jq --raw-output | less -S

[github-actions]

6690 tests run: 6356 passed, 0 failed, 334 skipped (full report)

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Flaky tests (2)

Postgres 17

• test_metrics_normal_work: release-arm64
• test_timeline_archival_chaos: release-x86-64

Test coverage report is not available

_{The comment gets automatically updated with the latest test results
a08f49f at 2025-01-21T00:29:31.283Z :recycle:}