Add `deck gateway apply` command #3275

Job	Run time
integration (kong:2.8)	2m 39s
integration (kong:3.4)	2m 40s
integration (kong:3.5)	2m 50s
integration (kong:3.6)	2m 41s
integration (kong:3.7)	2m 48s
integration (kong:3.8)	2m 48s
integration (kong/kong:master)	2m 50s
	19m 16s

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