Sitemaps abilities: register status reads + rebuild dispatch via Registrar (#48767)

* Sitemaps abilities: register status reads + rebuild dispatch via Registrar

* Sitemaps abilities: use core site category

* Sitemaps abilities tests: register core site category in lifecycle helper

* Sitemaps abilities: simplify get-status to reflect the served sitemap.xml (drop dup URL / synthetic last-build / reserved error; add real sitemaps[])

* Sitemaps abilities: pass string '0' to jp_sitemap_filename() to satisfy Phan type contract

* Sitemaps abilities: return next_scheduled_at (next jp_sitemap_cron_hook tick, UTC) from request-rebuild

* Sitemaps abilities: format next_scheduled_at as ISO 8601 UTC (Z suffix) for unambiguous timezone

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Co-authored-by: Enej Bajgoric <enej.bajgoric@a8c.com>

Author: enejb

projects/plugins/jetpack/changelog/add-sitemaps-abilities

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+Significance: minor
+Type: enhancement
+
+Abilities API: register sitemaps reads + rebuild dispatch

projects/plugins/jetpack/changelog/add-sitemaps-abilities-core-site-category

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+Significance: patch
+Type: other
+
+Sitemaps abilities: use the core site ability category

projects/plugins/jetpack/changelog/sitemaps-abilities-status-shape

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+Significance: patch
+Type: other
+
+Sitemaps abilities: simplify get-status output — drop the duplicate master_sitemap_url and the reserved last_error, replace the state-derived last_build_at with a `sitemaps` list reflecting the child sitemaps actually present in the served sitemap.xml (each with its own lastmod)