feat: Add single-seller feature, which is mutually exclusive with multiple-sellers #2149

Job	Run time
tests (controlled, all-features, both)	1m 25s
tests (controlled, all-features, approval)	1m 21s
tests (controlled, facilityuse-has-slots, approval)	1m 22s
tests (controlled, all-features, simple)	1m 22s
tests (controlled, facilityuse-has-slots, simple)	1m 21s
tests (controlled, no-auth, approval)	1m 24s
tests (controlled, no-auth, simple)	1m 21s
tests (controlled, no-payment-reconciliation, approval)	1m 23s
tests (controlled, no-payment-reconciliation, simple)	1m 28s
tests (controlled, no-tax-calculation, approval)	1m 31s
tests (controlled, prepayment-always-required, approval)	1m 20s
tests (controlled, no-tax-calculation, simple)	1m 22s
tests (controlled, prepayment-always-required, simple)	1m 22s
tests (controlled, single-seller, approval)	1m 20s
tests (controlled, single-seller, simple)	1m 20s
tests (random, all-features, approval)	1m 33s
tests (random, all-features, both)	1m 21s
tests (random, all-features, simple)	1m 22s
tests (random, facilityuse-has-slots, approval)	1m 20s
tests (random, facilityuse-has-slots, simple)	1s
tests (random, no-auth, approval)	1s
tests (random, no-auth, simple)	1s
tests (random, no-payment-reconciliation, simple)	1s
tests (random, no-payment-reconciliation, approval)	1s
tests (random, no-tax-calculation, approval)	1s
tests (random, no-tax-calculation, simple)	1s
tests (random, prepayment-always-required, approval)	1s
tests (random, prepayment-always-required, simple)	1s
tests (random, single-seller, approval)	1s
tests (random, single-seller, simple)	1s
deploy-output	0s
	26m 29s

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