Request Limiter reloadable config (#25095)

This commit introduces a new reloadable stanza to the server config to allow disabling the Request Limiter.

internalshared/configutil/merge.go

@@ -98,5 +98,10 @@ func (c *SharedConfig) Merge(c2 *SharedConfig) *SharedConfig {
 		result.ClusterName = c2.ClusterName
 	}
 
+	result.RequestLimiter = c.RequestLimiter
+	if c2.RequestLimiter != nil {
+		result.RequestLimiter = c2.RequestLimiter
+	}
+
 	return result
 }