Add option to always use known symbol servers for embedded and Source Link navigation

Part of https://github.com/dotnet/roslyn/issues/55834

Embedded source/Source Link navigation won't work without having the user opt in to some symbol servers, which are not defaulted. The debugger API allows us to ignore these defaults, so we can add a Roslyn option for "Always use known symbol servers for navigation" and default it to on, to improve discoverability of the feature.