draft-core-yang-library: Unconstrained YANG Library reference

[vvilimek]

Add one new leaf referencing RFC8525 YANG Library resource.

If the constrained node does not implement any 'location' list for each one module (as recommended by the text), we want to allow the node to have one string referencing the 'unconstrained' YANG Library. This way, human discovery of supported nodes improve greatly for little to no penalty.

[vvilimek]

We might even create an augmentation of the ietf-yang-library/yang-library, something like:

module draft-yang-library {
  yang-version 1.1;
  namespace "http://example.com/ietf/core/yanglib";
  prefix cyl;

  import ietf-yang-library {
    prefix yanglib;
    revision 2019-01-04;
    reference "RFC8525: YANG Library";
  }

  import ietf-inet-types {
    prefix inet;
    reference "RFC9911";
  }

  augment "/yanglib:ietf-yang-library/yanglib:yang-library" {
    leaf constrained-yang-library {
      type inet:uri;
      description "Link to constrained YANG Library resource.";
    }
  }
}