Add dkan_datastore_preview datastore submodule

[dcgoodwin2112]

Fixes [issue#]

Describe your changes

Adds dkan_datastore_preview, a submodule of dkan_datastore that renders a paginated, sortable HTML table of datastore data for each CSV/TSV distribution on the server-rendered dataset page (/node/{nid}). MVP scope from the ticket: find the datastore table and display it in the dkan_metastore twig template; the block and a configurable data-source plugin system are follow-ups.

• DataSourceInterface + DatabaseDataSource (seam for a future plugin system), DataPreviewBuilder, dkan_datastore_preview render element, ImportStatusMessage
• Server-rendered only: sort links and page-size form use per-table prefixed query params (dp0_order, dp1_page_size, …) so multiple previews on one page don't share state; no JavaScript
• Shows a status message instead of nothing when a resource isn't imported yet; distribution cache tags flip it to the table after import without a cache rebuild
• node--data.html.twig prints {{ content.data_preview }} (empty when the module is disabled; dkan_metastore gains no dependency on dkan_datastore)
• ddev dkan-site-install enables the module

QA Steps

Prerequisite: a dataset with one imported CSV distribution (drush dkan:datastore:list shows done), enough rows to page (more than 25).

• drush en dkan_datastore_preview, then open the dataset's node page (/node/{nid}, not the React /dataset/{uuid} page). A captioned preview table renders below Downloads with "Showing 1-25 of N results".
• Click a column header. URL gains dp0_order/dp0_sort, the header gets aria-sort, rows re-sort. Click again to reverse.
• Set page size 10 and Apply. "Showing 1-10 of N", a pager appears, and pager links keep dp0_page_size=10.
• Open a dataset whose distributions are not CSV/TSV (or has none). No preview section, no errors in drush watchdog:show.
• drush dkan:datastore:drop <identifier> (identifier from drush dkan:datastore:list), drush cr, reload: "Data preview is not yet available…" message instead of the table.
• drush dkan:datastore:import <identifier> && drush queue:run datastore_import, reload without drush cr: the table is back.
• drush pmu dkan_datastore_preview, reload: page renders normally with no preview and no errors.

Checklist before requesting review

If any of these are left unchecked, please provide an explanation

• I have updated or added tests to cover my code
• I have updated or added documentation — no module README yet; will add with the block/plugin follow-up unless reviewers want it here

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