Add subject type #3603

Job	Run time
check (examples)	2m 3s
check (docs)	1m 8s
check (format)	17s
check (licenses)	1m 30s
check (lint)	1m 20s
check (msrv)	2m 11s
check (spelling)	7m 54s
test (ubuntu-latest, nats-main)	13m 51s
test (ubuntu-latest, nats-dev)	12m 42s
test (macos-latest, nats-main)	14m 3s
test (macos-latest, nats-dev)	11m 45s
test (windows-latest, nats-main)	22m 0s
test (windows-latest, nats-dev)	18m 14s
	1h 48m 58s

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