Convert rolegroup configuration to use the new fragment infrastructure

[nightkr]

Description

This is an example of a conversion to use stackabletech/operator-rs#445 for typed access to the rolegroup configuration.

Currently the product-config generator accesses the ZookeeperConfigFragment directly and does its own merging, but ideally this would be moved to use the merged ZookeeperConfig format eventually. This was kept out of scope for now to get what we have into a releasable state.

Review Checklist

• Code contains useful comments
• CRD change approved (or not applicable)
• (Integration-)Test cases added (or not applicable)
• Documentation added (or not applicable)
• Changelog updated (or not applicable)
• Cargo.toml only contains references to git tags (not specific commits or branches)
• Helm chart can be installed and deployed operator works (or not applicable)

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