refactor: Wrap SparkApplicationSpec in SparkApplicationTemplateSpec

[sbernauer]

Description

Follow-up of #660, I accidentally stumbled across this when adding testdata.

To quote claude:

• Maintenance bomb. Every new field has to be added in three places: both structs and the From impl. There is nothing in the type system that flags a forgotten one — you'd get a silently-dropped field in templates.
• The From impl is dead code waiting to happen. It exists only because the two types are nominally distinct. If they were the same type (or template wrapped/reused the spec), you'd just have spec.clone().

Instead we express what SparkApplicationTemplateSpec is: Just a wrapper around SparkApplicationSpec.

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

[sbernauer]

--- PASS: kuttl (291.79s)
    --- PASS: kuttl/harness (0.00s)
        --- PASS: kuttl/harness/spark-history-server_openshift-false_spark-4.1.1_s3-use-tls-true (291.78s)