Get cache checksum from server when repository list is fetched

Qt Installer downloads each repository's Updates.xml in startup and in component selection view when new category is fetched. Checksums of each repositorys Updates.xml files are compared to the local cached items to check if the repository in question should download new version of metadata.

Qt Installer gets its repository list from server. This repository list could contain also checksums of Updates.xml files. We could use this checksum to check if the cached items have changed instead of downloading each Updates.xml separately for checking if the metadata has changed and needs reload.

Example format of the repository could be:

"repositories": [        
        {
            "action": "add",
            *"checksum-sha1":"12345678890"*,
            "categories": [
                "Long Term Support",
                "Latest releases",
                "Archive"
            ],
            "name": "Qt from my repository",
            "releasedate": "2022-10-07T11:00:00.000Z",
            "url": "https://my_repository"
        }