Align Pyserini REST API with Anserini REST API

[lintool]

We already have REST APIs in both Pyserini and Anserini.

The one in Anserini is more refined, complete with openAPI spec: https://github.com/castorini/anserini/blob/master/src/main/resources/rest/openapi.yaml

We should bring the Pyserini REST API up to speed, and ensure that changes are propagated over the MCP.