Make "tag" and "id" required properties in YAML metaschema

[drdavella]

This seems reasonable to me but may require some of the schemas to be modified slightly in order to conform. There may be reasons I'm overlooking for why this isn't going to work, though.

[drdavella]

It's possible that tag should not be mandatory since not all schemas end up being associated with YAML tags. For example, a schema may only be included as a reference within another schema.

[braingram]

Using a root-level tag is problematic (see: https://github.com/asdf-format/asdf-standard/issues/269). Requiring id (and checking that it matches the uri registered with asdf) seems beneficial.