Add link@rel=via

[prushforth]

Per @joanma747 Testbed 13 recommendations, it may be useful to add spec and implementation support for link@rel=via to support linking the map layers to layer metadata. Thanks to @AmeliaBR for suggesting we get serious about discussing / accepting / rejecting Testbed report recommendations here/elsewhere.

Note I will be adding issues here and where accepted recommendations were made but neglected to do anything about them. This is not the final word! Just putting this issue here for discussion.