Server app currently does not support dense retrieval indexes

[lintool]

The current webapp/REST API https://github.com/castorini/anserini/blob/master/docs/rest-api.md does not support dense HNSW indexes.

For example, this works, on msmarco-v1-passage:

curl -X GET "http://localhost:8081/api/v1.0/indexes/msmarco-v1-passage/search?query=How%20does%20the%20process%20of%20digestion%20and%20metabolism%20of%20carbohydrates%20start" 

But this doesn't:

curl -X GET "http://localhost:8081/api/v1.0/indexes/msmarco-v1-passage.bge-base-en-v1.5.hnsw/search?query=How%20does%20the%20process%20of%20digestion%20and%20metabolism%20of%20carbohydrates%20start" 

That's because msmarco-v1-passage.bge-base-en-v1.5.hnsw uses an HNSW index, and https://github.com/castorini/anserini/blob/master/src/main/java/io/anserini/server/SearchService.java does not provide bindings to HNSW indexes.

We should start by fixing this.

@vincent-4