Add permission to list all installed Snaps

Currently, a website can only list Snaps that the website is connected to (i.e., has the `wallet_snap` permission for). For the Snaps Directory (and other 3rd party directories), it can be useful to list all installed Snaps, using a method like `wallet_getAllSnaps`. This method should require a permission, and only be usable by websites.

Start by tracing the existing wallet_snap permission handling and wallet_* methods, especially the proposed wallet_getAllSnaps entry point. Define the permission required for listing every installed Snap and ensure the method is restricted to websites; the issue is complete when this behavior is implemented and covered by the relevant tests.

Written by the indexing model from the issue text.