Add more metadata fields for mapping schemas

After taking a look on how the id, title, and description fields of the mapping schema records are currently used, it has become clear that the following fields are missing:
- createdAt
- lastUpdatedAt
- version
- link to further resources (e.g., the mapping schema in metastore/base-repo)

Start by locating the mapping schema records and the existing handling of their id, title, and description fields. Trace how these records are represented and exposed through the RESTful API, then determine where the four requested metadata fields belong. Done means the API consistently supports createdAt, lastUpdatedAt, version, and a link to further resources.

Written by the indexing model from the issue text.