Indexers: `/clear`

Add a `/clear` endpoint to each Indexer that deletes all the documents stored in it.

See as an example [this implementation](https://github.com/jina-ai/executors/blob/fa30ca17aaf805fc505de552a58c58c2459562be/jinahub/indexers/searcher/HnswlibSearcher/hnswlib_searcher.py#L259)

NOTE: If you want to contribute to this, make sure to create a sub-task for the specific Indexer you want to work on for this endpoint and add it here. Don't close this entire issue, as it related to *multiple* Indexers.