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

config/constants.go

@@ -37,6 +37,7 @@ var ViperPolicyHooks = viper.DecodeHook(mapstructure.ComposeDecodeHookFunc(
 	// parse base-64 encoded POLICY that is bound to environment variable
 	DecodePolicyBase64Hook(),
 	decodeNullBoolHookFunc(),
+	decodeNullUint32HookFunc(),
 	decodeJWTClaimHeadersHookFunc(),
 	decodeBearerTokenFormatHookFunc(),
 	decodeCodecTypeHookFunc(),