Throttler: `CheckThrottlerResponseCode` to replace HTTP status codes #345

Job	Run time
Set push variable	1s
Set push variable	1s
Build and push vttestserver (mysql80)	2m 7s
Build and push vttestserver (mysql80)	2m 7s
Build and push lite (latest)	2m 15s
Build and push lite (latest)	2m 15s
Build and push lite (percona80)	31m 11s
Build and push lite (percona80)	2m 11s
Build and push	0s
Build and push (bullseye, vtadmin)	1m 22s
Build and push (bullseye, vtorc)	41s
Build and push (bullseye, vtgate)	39s
Build and push (bullseye, vttablet)	31s
Build and push (bullseye, mysqlctld)	47s
Build and push (bullseye, mysqlctl)	41s
Build and push (bullseye, vtctl)	33s
Build and push (bullseye, vtctlclient)	31s
Build and push (bullseye, vtctld)	1m 10s
Build and push (bullseye, vtctldclient)	37s
Build and push (bullseye, logrotate)	32s
Build and push (bullseye, logtail)	20s
Build and push (bullseye, vtbackup)	41s
Build and push (bullseye, vtexplain)	41s
Build and push (bookworm, vtadmin)	1m 12s
Build and push (bookworm, vtorc)	40s
Build and push (bookworm, vtgate)	33s
Build and push (bookworm, vttablet)	44s
Build and push (bookworm, mysqlctld)	35s
Build and push (bookworm, mysqlctl)	34s
Build and push (bookworm, vtctl)	44s
Build and push (bookworm, vtctlclient)	47s
Build and push (bookworm, vtctld)	1m 14s
Build and push (bookworm, vtctldclient)	24s
Build and push (bookworm, logrotate)	18s
Build and push (bookworm, logtail)	24s
Build and push (bookworm, vtbackup)	30s
Build and push (bookworm, vtexplain)	33s
	1h 1m 6s

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