SLR: Add filter_exclusive property to tool manifests

For "pure" container runtimes (soldier, sniper and future branches) we set a low priority, so that these container runtimes will be hidden from the Steam UI, unless forced via an intentionally undocumented developer option. For simplicity, if scout is built as a pure container runtime, it gets the same treatment (but in practice we do not generate or publish a pure scout runtime any more, because "layered" scout-on-soldier turned out to have significantly better compatibility with real-world games).

For "layered" container runtimes (scout-on-soldier, and possibly scout-on-sniper in future), we set a higher priority, so that these container runtimes will be shown for legacy scout Linux games.

For games that target a later ABI, such as CS2 targeting sniper, the Steam client is expected to overwrite the priority value set here with a priority greater than 10, so that the appropriate runtime will be the only one that is offered.

steamrt/tasks#426