Implement Redis storage backend

[AnneYang720]

Now Redis supports vector similarity search algorithm based on HNSW. #452

This issue will declare the overall steps on supporting a new vector storage backend: Redis into docarray.

This is part of Summer Of Open Source project work with Jina from students, together with @bwanglzu. To achieve the goal, we break down our tasks in to the following sub-tasks:

• [ ] implement backend.py
• [ ] implement getsetdel.py
• [ ] implement seqlike.py
• [ ] implement find.py
• [ ] documentation on the usage of RedisStorage in integrations/document store page.
• [ ] benchmarking and compare with other storage backends.
• [ ] document benchmarking results.