Support multiple source sites

Content Diff can presently handle just one source site being merged to local site. Add support multiple source sites merged into one target site.

The way to go is probably just by adding a new `--source-site-domain` argument to the import command, to distinguish multiple sites. And it would most likely be enough to add just one more meta,
- the existing [ID mapping meta_key](https://github.com/Automattic/newspack-content-diff-migrator/blob/trunk/src/Logic/ContentDiffLogic.php#L25) is `newspackcontentdiff_live_id`
- so perhaps add one more postmeta/usermeta, e.g. `newspackcontentdiff_live_domain` to note the source site

Start with src/Logic/ContentDiffLogic.php, especially the existing newsspackcontentdiff_live_id mapping, and trace the import command's source-site handling. Define how --source-site-domain and the proposed domain metadata distinguish records from multiple source sites, then verify that several sources can merge into one target without collisions.

Written by the indexing model from the issue text.