feat(indexer): Improve `HttpRestKvClient` with multi-get functionality

Following up on https://github.com/iotaledger/iota/issues/9410, we should modify implementation of the client so that it uses [multi-get](https://github.com/iotaledger/iota/issues/9413) requests in `multi_fetch` instead of calling `fetch` multiple times in a loop.