Commit Message: Replace rapidjson with nlohmann/json
Additional Description:

• Schema validation is not supported by the core nlohmann/json library. They endorse an option here json schema validator nlohmann/json#305 but schema validation is not used anywhere in Envoy core.
• Added runtime option envoy.reloadable_features.remove_legacy_json. It is disabled by default. I will enable the new parser in a separate PR.
• There is one usage in core to parse metadata in the HeaderFormatter:

  envoy/source/common/router/header_formatter.cc

  Line 61 in e073b32

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

  This could be replaced pretty easily with protobuf's parser in a later PR
• Originally I had an implementation that shared the SAX parser between the two implementations, but RapidJson and nlohmann/json have to handle tracking line numbers differently so I split them. (RapidJSON makes it easy to subclass the stream implementation, but nlohmann/json doesn't.)
• The major difference between the two json_internal.cc implementations is the custom stream class (see json_loader.cc line 243-280. It has a reference to the handler to update line numbers)

Risk Level: Low for core, maybe medium for those using JSON on the dataplane.
Release Notes: Added release notes about runtime feature
Testing:

• json_loader_test.cc is parametrized over both runtime values.
  Fixes Remove RapidJSON from Envoy #4705