Seed support in YML format

**Actual Behaviour**
Currently it is required to create seeds in json format.

**Expected Behaviour**
It would be better if the plugin can also support seeds in yml format.

The issue names no files or tests. Start by tracing how the plugin discovers and parses JSON seed files, then identify the corresponding seed-loading entry point and existing test coverage. Done means valid YML seed files are accepted alongside JSON without breaking the current JSON behavior, with tests covering both formats.

Written by the indexing model from the issue text.