config: add circuit breaker thresholds (#5650)

## Summary
Add a new `circuit_breaker_thresholds` option:

```yaml
circuit_breaker_thresholds:
  max_connections: 1
  max_pending_requests: 2
  max_requests: 3
  max_retries: 4
  max_connection_pools: 5
```

This option can be set at the global level or at the route level. Each
threshold is optional and when not set a default will be used. For
internal clusters we will disable the circuit breaker. For normal routes
we will use the envoy defaults.

## Related issues
-
[ENG-2310](https://linear.app/pomerium/issue/ENG-2310/add-circuit-breaker-settings-per-route)

## Checklist
- [x] reference any related issues
- [x] updated unit tests
- [x] add appropriate label (`enhancement`, `bug`, `breaking`,
`dependencies`, `ci`)
- [x] ready for review

authorize/log_test.go

@@ -85,7 +85,7 @@ func Test_populateLogEvent(t *testing.T) {
 		{log.AuthorizeLogFieldQuery, s, `{"query":"a=b"}`},
 		{log.AuthorizeLogFieldRemovedGroupsCount, s, `{"removed-groups-count":42}`},
 		{log.AuthorizeLogFieldRequestID, s, `{"request-id":"REQUEST-ID"}`},
-		{log.AuthorizeLogFieldRouteChecksum, s, `{"route-checksum":14294378844626301341}`},
+		{log.AuthorizeLogFieldRouteChecksum, s, `{"route-checksum":7416256365460802121}`},
 		{log.AuthorizeLogFieldRouteID, s, `{"route-id":"POLICY-ID"}`},
 		{log.AuthorizeLogFieldServiceAccountID, sa, `{"service-account-id":"SERVICE-ACCOUNT-ID"}`},
 		{log.AuthorizeLogFieldSessionID, s, `{"session-id":"SESSION-ID"}`},