api: enum proposal states

[lukaw3d]

Based on

nexus/coreapi/v23.0/governance/api/proposal.go

Lines 22 to 25 in dc95e6b

	StateActiveName = "active"
	StatePassedName = "passed"
	StateRejectedName = "rejected"
	StateFailedName = "failed"

[mitjat]

Thank you. I extracted the enum into an explicitly named type and also used it on the query side (where you can specify which proposals you want to retrieve).