adding custom hook for generated xml properties

[elysium001]

Feature: Sitemap Generation Hook

Description:

This pull request introduces a new action hook, msm_sitemap_generated, triggered after a sitemap has been generated. This hook provides developers with a convenient entry point to perform additional actions, such as pinging search engines, after the creation of a sitemap.

This enhancement provides flexibility for developers to extend and customize the sitemap generation process based on their specific needs.

UPDATE
After further review, appending the xml and total_url_count values to the existing hooks msm_update_sitemap_post and msm_insert_sitemap_post makes the most sense.

[GaryJones]

Instead of adding a new hook, could the generated XML and total_url_count not just be added to the msm_update_sitemap_post and msm_insert_sitemap_post hook instances, since they happen right before the proposed new action hook spot?