Support "tags" yaml tag
_The documentation is not available anymore as the PR was closed or merged._ IMO `DatasetMetadata` shouldn't crash with attributes that it doesn't know, btw Yea this PR is mostly to have a validation that this field contains a list of strings.

Regarding unknown fields, the tagging app currently returns an error if a field is unknown using the `DatasetMetadata`. We can change that though
Added the "tags" YAML tag, so that users can specify data domain/topics keywords for dataset search