Add `BulkImports::NetworkError` to wrap network exceptions from
`BulkImports::Client::Graphql` and `BulkImports::Client::HTTP`. The
wrapper has a `#retriable?` method that returns `true` when the
exception is something worth retrying, like a timeout, and didn't happen
more than `BulkImports::NetworkError::MAX_RETRIABLE_COUNT`, currently 3.
The counter is scoped by:
- the exception type;
- `BulkImports::Tracker`;
- `BulkImports::Entity` and;
- `BulkImports::Stage`.

Changelog: added
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68582