[client.Client] Move autoreject logic to Go

[willbeason]

Prerequisites: #193 , #176

Per benchmarks in #186 , autorejection has a massive performance impact since (1) duplicating objects in Rego is expensive and (2)

Sample comparison. Each test indicates the result of every Constraint, so "autoreject" means all 1,000 Constraints autorejected the request.

BenchmarkClient_Review/1000_Constraints_100_Templates_success-12             	     122	  98551484 ns/op	26344853 B/op	  572764 allocs/op
BenchmarkClient_Review/1000_Constraints_100_Templates_fail-12                	      55	 201748681 ns/op	58626552 B/op	 1137203 allocs/op
BenchmarkClient_Review/1000_Constraints_100_Templates_filtered_out-12        	     415	  29254883 ns/op	 7601835 B/op	  179334 allocs/op
BenchmarkClient_Review/1000_Constraints_100_Templates_autoreject-12          	      31	 347175043 ns/op	115088467 B/op	 2127332 allocs/op

What's convenient for transitioning to a Go implementation of autorejection is that this means we can prevent all Constraints from running if even one autorejects. Autorejection means we are operating in an inconsistent state and will deny the object in the review request either way.

[willbeason]

Autorejection should happen in the Client layer when it runs matching logic. This means we'll autoreject in Go before executing any Rego.