Add extendable filter categories

**Is your feature request related to a problem? Please describe.**
As of now, MLX can't replicate MAX/DAX as fields in the metadata are missing

**Describe the solution you'd like**
Please add the following meta data field to all asset types:
- `metadata` of type [`JSON`](https://dev.mysql.com/doc/refman/5.7/en/json.html#json-paths)

**Describe alternatives you've considered**
Alternatively, metadata for each asset type is static which would make MLX highly inflexible to adjust to the needs of different organizations

**Related work items:**
- [x] API: https://github.com/machine-learning-exchange/mlx/pull/183
- [ ] UI: @drewbutlerbb4
- [ ] Katalog: https://github.com/machine-learning-exchange/katalog/pull/16