Add generativeUri to metadata :)

**Describe the feature**
[Since we have `generativeUri` in content](https://github.com/kodadot/packages/blob/main/hyperdata/tests/generative.test.ts) It would be nice to add it to the indexer 

**Expected outcome**

1. Extend `MetadataEntity` with `generativeUri`
2. export it from the `metadata` fn


**Additional context**
- https://github.com/kodadot/private-workers/pull/114
