Add external function library #7294

Job	Run time
lint/style-and-typos	1m 21s
linux/3.9/bare-env	15s
linux/3.9	30m 7s
linux/3.12	31m 48s
osx/3.9	21m 23s
linux/3.13	31m 23s
linux/3.11	26m 17s
osx/3.10	23m 22s
osx/3.13	20m 58s
osx/3.12	23m 1s
win/3.9	45m 59s
osx/3.11	23m 18s
linux/3.10	28m 57s
win/3.9/pip	21m 18s
linux/3.11/conda	24m 13s
win/3.12	53m 8s
linux/3.10/mpi	9m 56s
win/3.13	49m 50s
linux/3.11/singletest	31m 47s
win/3.10	2h 0m 16s
linux/3.9/slim	4m 57s
linux/3.12/cython	24m 13s
linux/3.10/pyutilib	17m 9s
linux/3.12/numpy2	6m 55s
win/3.11	49m 24s
linux/pypy-3.10	46m 31s
linux/3.11/rtd	29m 19s
process-coverage-${{ matrix.TARGET }}	0s
	13h 17m 5s

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