Set default Certificate exchange config in local devnet manifest (#475)

Set the default cert exchange configuration in the local devnet
manifest, since nil cert exchange config causes panic.

Relates to: filecoin-project/lotus#12236

manifest/manifest.go

@@ -139,6 +139,7 @@ func LocalDevnetManifest() *Manifest {
 		BootstrapEpoch: 1000,
 		EcConfig:       DefaultEcConfig,
 		GpbftConfig:    DefaultGpbftConfig,
+		CxConfig:       DefaultCxConfig,
 	}
 	return m
 }