params: clarify consensus engine config Strings (ethereum#29643)

Define these on a value receiever so that nil is shown differently.

params/config.go

@@ -374,7 +374,7 @@ type ChainConfig struct {
 type EthashConfig struct{}
 
 // String implements the stringer interface, returning the consensus engine details.
-func (c *EthashConfig) String() string {
+func (c EthashConfig) String() string {
 	return "ethash"
 }
 
@@ -385,8 +385,8 @@ type CliqueConfig struct {
 }
 
 // String implements the stringer interface, returning the consensus engine details.
-func (c *CliqueConfig) String() string {
-	return "clique"
+func (c CliqueConfig) String() string {
+	return fmt.Sprintf("clique(period: %d, epoch: %d)", c.Period, c.Epoch)
 }
 
 // Description returns a human-readable description of ChainConfig.