Remove unused methods and exceptions from JsonObject

[asraa]

1. Remove instance of loadFromString in header formatter and replace with protobuf .

   envoy/source/common/router/header_formatter.cc

   Line 61 in e073b32

   Json::ObjectSharedPtr parsed_params = Json::Factory::loadFromString(std::string(json));

2. Remove unused methods and the unsafe code that can throw exceptions such as the get[Type] methods that don't have defaults.

   envoy/include/envoy/json/json_object.h

   Line 114 in 0d5b470

   virtual std::string getString(const std::string& name) const PURE;

Follow-up to #14467