Add Carbon (BuySellAds) ad provider for waiting room #6220

Job	Run time
build-and-check	2m 58s
test-integration-packages/internal	1m 15s
test-integration-packages/agent-runtime	44s
test-integration-sdk	1m 21s
test-integration-web	42s
test-integration-common	47s
test-packages/agent-runtime	47s
test-integration-cli	51s
test-packages/billing	51s
test-integration-packages/billing	1m 7s
test-web	54s
test-.agents	46s
test-packages/internal	49s
test-sdk	42s
test-common	46s
test-cli	54s
	16m 14s

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