Add `deck gateway apply` command #3270

Job	Run time
integration (kong:2.8)	2m 39s
integration (kong:3.4)	2m 51s
integration (kong:3.5)	2m 51s
integration (kong:3.6)	2m 42s
integration (kong:3.7)	2m 51s
integration (kong:3.8)	2m 57s
integration (kong/kong:master)	2m 48s
	19m 39s

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