Support for DTNMA ADM modules in YANG syntax

[BrianSipos]

The internet draft in ietf-dtn-adm-yang defines an alternative use for YANG modules to encode DTNMA Application Data Models (ADMs). This adds to the guidelines from RFC 8407 and has implications for some of the default linter warnings from pyang.

The ADM module is still in draft, so there can be future changes affecting the lint behavior, but the basic structure has been stable for some time now. The first issue with the current linter is the limitation that module namespaces begin with the IETF prefix urn:ietf:params:xml:ns:yang: which will need to be relaxed to include ari:// prefix also.