New generic metadata attribute

[robander]

First suggested by Eliot Kimber as a way to handle metadata concerns: a new base attribute (to be named), possibly with a URI value or possibly free-form, that can be used to specify or refer to metadata values defined outside of the DITA source (such as an externally maintained taxonomy).

Original idea came out of earlier threads and meeting discussion; first discussed as a stage 1 proposal January 23, 2018, based on summary of recent metadata notes here: https://lists.oasis-open.org/archives/dita/201801/msg00048.html