Expose `fetch` and `fetch_by_key` functions on the new-style `Connection` API

[da-tanabe]

Expose analogous methods to https://docs.daml.com/app-dev/bindings-ts/daml-ledger/index.html#fetch and https://docs.daml.com/app-dev/bindings-ts/daml-ledger/index.html#fetchbykey in dazl.