python: use plugins if available in the jobspec validator #3227

Job	Run time
spelling	4s
validate commits	0s
python linting	32s
Generate build matrix	7s
bookworm - test-install	11m 51s
bookworm - 32 bit	12m 54s
noble - test-install	12m 53s
el9 - test-install	12m 3s
alpine - test-install	12m 25s
fedora40 - test-install	11m 55s
jammy - test-install	12m 4s
focal - py3.8	16m 49s
bookworm - gcc-12,content-s3,distcheck	19m 39s
fedora40 - clang-18	16m 55s
coverage	23m 45s
el8 - ascii	19m 28s
el8 - test-install	13m 3s
el8 - system,coverage	7m 39s
inception	22m 2s
flux-pmix check	13m 22s
macos build only	2m 39s
flux-pam check	4m 24s
flux-accounting check	4m 31s
flux-sched check	6m 51s
Generate docker manifest	1s
	4h 17m 56s

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