Library use: Enable setting HTTP User Agent more conveniently

## About
For library use, it would be convenient if the downstream application could set the HTTP User Agent more conveniently than needing to [override and monkeypatch the `croud.api.Client` class](https://github.com/crate/cratedb-toolkit/blob/86e712cef482ad9298fad0f344369011c9f3cdcd/cratedb_toolkit/util/croud.py#L213-L224).
