oxide_ip_pool_silo_link: use composite attributes for id #1859

Job	Run time
omicron-version	10s
build-test (terraform)	2m 53s
build-test (tofu)	1m 51s
publish-image	7s
acceptance / acceptance (terraform)	6m 48s
acceptance / acceptance (tofu)	6m 30s
	18m 19s

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