ratelimits: Add a feature-flag which makes key-value implementation authoritative #6829

Job	Run time
vendorcheck (1.22.5)	19s
govulncheck	17s
b (go1.22.5_2024-07-03, ./t.sh --lints --generate)	3m 15s
b (go1.22.5_2024-07-03, ./t.sh --integration)	5m 51s
b (go1.22.5_2024-07-03, ./tn.sh --integration)	8m 9s
b (go1.22.5_2024-07-03, ./t.sh --unit --enable-race-detection)	7m 32s
b (go1.22.5_2024-07-03, ./tn.sh --unit --enable-race-detection)	7m 31s
b (go1.22.5_2024-07-03, ./t.sh --start-py)	2m 47s
Boulder CI Test Matrix	1s
	35m 42s

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