add W3C baggage support

https://w3c.github.io/baggage/

Along with 'traceparent' and 'tracestate', we could have the Node.js APM agent (and, of course, the other agents):

1. propagate the `baggage` header; and
2. provide APIs to add/get/remote baggage

At time of writing, the baggage spec is still a "First Public Working Draft (FPWD)" so priority is probably lower. One concrete possible use case (not definitive yet) is this Kibana issue -- https://github.com/elastic/kibana/issues/97934 -- to attempt to propagate info on a Kibana action (front end or back end) all the way through to an Elasticsearch slowlogs entry. https://github.com/elastic/kibana/issues/97934#issuecomment-854257718 is a description of the idea.