Title: Evaluate upgrade path from RequestInfo::dynamicMetadata to new DynamicMetadata class

Description:

As part of working on implementing a new DynamicMetadata class for arbitrarily typed communication between filters, I've been evaluating current uses of RequestInfo::dynamicMetadata(). After some discussion with @htuch, I wanted to poll the experts on the current uses of that method about whether they think an upgrade to the new method is feasible. Would each of you be willing to take a look at the design and comment as to whether you think there's a reasonable deprecation path from the old method to the new one, whether the two methods should be kept on in parallel, or if you have another preference? (The old method is an arbitrary proto indexed by RDNS filter name, the new method is an arbitrary C++ object indexed by RDNS organization+data name.)

Lua filter: @dio
RBAC filter: @yangminzhu
HeaderToMetadata: @rgs1
FYI: @mattklein123 @alyssawilk

Relevant Links
Design doc: https://docs.google.com/document/d/1GNawM59Pp09WZ34zqJYiO_qmERjuGbOUeAqF1GG6v9Q/edit#
(The key section for this issue is "Accessing data produced in one filter from another")

Implementation PR: #3918