api: make num_threads configurable #45621

Job	Run time
api-check-needed	6s
Lint: ClientPrint (ESLint)	16s
e2e-check-needed	5s
frontend-check-needed	4s
Tests: PDF	21s
Lint: Print (ESLint)	22s
Tests: Print	48s
Tests: API	4m 48s
Lint: API (php-cs-fixer)	33s
Lint: e2e (ESLint)	20s
Tests: End-to-end build / Tests: End-to-end (build job)	2m 19s
Lint: Frontend (ESLint)	30s
Tests: Frontend	3m 42s
Tests: End-to-end run / Tests: End-to-end (chrome)	4m 8s
Tests: End-to-end run / Tests: End-to-end (firefox)	4m 44s
Tests: End-to-end run / Tests: End-to-end (edge)	4m 14s
Finish coveralls report	2s
workflow-success	2s
Send out CI success event	0s
	27m 24s

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