When installing a release, install operators in parallel

[nightkr]

Description

stackablectl release install is sloooow, partially because we wait for each operator to download/start before we start installing the next one.

We might as well try to install them in parallel.

Ideally each Helm operation would run on a background task (via tokio's spawn_blocking), but that's a much larger refactoring than this PR.

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

# Author
- [x] Helm chart can be installed and deployed operator works

# Reviewer
- [ ] Code contains useful comments
- [ ] (Integration-)Test cases added
- [ ] Documentation added or updated
- [ ] Changelog updated
- [ ] Cargo.toml only contains references to git tags (not specific commits or branches)

# Acceptance
- [ ] Feature Tracker has been updated
- [ ] Proper release label has been added

Once the review is done, comment bors r+ (or bors merge) to merge. Further information

[Techassi]

Nice work!