refactor: Introduce build aggregator

[adwk67]

Description

Part of stackabletech/issues#864.

• Plain KubernetesResources (5 fields, no generic); fallible build(cluster, sa_name) reusing existing builders
• Reconcile rewritten to a build-then-apply loop (StatefulSets last)
• The three secret calls isolated into ensure_random_secrets.
• No behavioural change.
• Built-SA name
• Executor helpers moved to controller/mod.rs.

TODO

• check internal secrets in deref step and apply later

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

• Changes are OpenShift compatible
• CRD changes approved
• CRD documentation for all fields, following the style guide.
• Helm chart can be installed and deployed operator works
• Integration tests passed (for non trivial changes)
• Changes need to be "offline" compatible
• Links to generated (nightly) docs added
• Release note snippet added

Reviewer

• Code contains useful comments
• Code contains useful logging statements
• (Integration-)Test cases added
• Documentation added or updated. Follows the style guide.
• 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
• Links to generated (nightly) docs added
• Release note snippet added
• Add type/deprecation label & add to the deprecation schedule
• Add type/experimental label & add to the experimental features tracker

[maltesander]

This is not quite true as discussed, we can switch to deref (-> validate) and build steps.