CLI: consider an introducing some idiom that allows succinct file extension checking

> nit: consider an introducing some idiom in the codebase that allows succinct file extension checking. Maybe it's a new LINQ method (`SingleExtension` or `WhereExtension`) or a helper method called in existing `LINQ` methods. Or a wrapper around the extension LINQ expressions. Seems a bit verbose as-is. You need to think about the StringComparison each time but really it should be an impl. detail of a helper (IMO. Just a thought!

_Originally posted by @joelverhagen in https://github.com/dotnet/sign/pull/471#discussion_r1029599504_