Sitemap: add ability to modify column

Please add a hook to modify sitemap's column. For example we want to add some data to sitemap or we want to remove lastmod from sitemap.

Its implemented in core WP with 'wp_sitemaps_posts_entry' hook.

Start by tracing Jetpack's sitemap generation and compare its entry handling with WordPress core's `wp_sitemaps_posts_entry` hook. Done means sitemap entries can be modified so callers can add data or remove `lastmod`, with the behavior covered by the relevant existing tests if available.

Written by the indexing model from the issue text.