[TASK] Refactor element dependency resolving

Dependency element resolving of nested data structures currently
is used in CommandMap only. However the logic can be used for
other tasks as well, like e.g. showing nested structures in the
workspaces module. Callback methods are extracted to the new
ElementEntityProcessor to reduce further code duplication.

Resolves: #57231
Releases: 6.2
Change-Id: If588e2c644fb897f0ac5f7aaf4f341ce8ac5c232
Reviewed-on: https://review.typo3.org/28700
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader