On-chain contact list

**Is your feature request related to a problem? Please describe.**
Since https://github.com/bitshares/bitshares-core/pull/1926, it is possible to implement an on-chain contact list feature.

**Describe the solution you'd like**

**Describe alternatives you've considered**

**Additional context**
The feature will likely come in the mobile app first. The reference UI can use the same data format.
Low priority at the moment. Just creating an issue to track the idea and discussions now.

Start by reviewing PR #1926 and the mobile-app/reference-UI context described here. The issue does not name an entry point, file, test, data format, or acceptance criteria, so those need clarification before implementation. Done would require an agreed on-chain contact-list scope and corresponding UI and data-format requirements.

Written by the indexing model from the issue text.