Add OAuth2 ClientCredentials flow support #923

Job	Run time
check-commits	18s
build / build	16s
lint / lint (3.11)	28s
lint / lint (3.12)	35s
docs / test	59s
docs / no-test	0s
codeql / Analyze	56s
test / test (nightly, /relocated/djnd/, true, 3.11)	1m 9s
test / test (latest, 3.11)	1m 8s
test / test (3.39, 3.12)	1m 7s
test / test (3.28, true, 3.12)	1m 8s
test / test (3.22, 3.8)	1m 8s
test / test (3.21, /relocated/djnd/, 3.9)	1m 7s
test / test (3.18, true, 3.10)	1m 8s
ready-to-ship	1s
	11m 28s

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