Allow export of full, raw ClickHouse data from Plausible Cloud

True migration between Plausible Cloud and self-hosted cannot be done because only daily summaries of visits are exported. Allow export of the full ClickHouse database with full granularity so all data can be migrated from Cloud to self-hosted. This would likely be a large file, so it can be made a paid option or limited to once per billing period. Import would be effortless to implement since it would simply overwrite the existing self-hosted ClickHouse database.