Tooltip suggestion - show "allow ref struct" for typars of external types and methods

This is a feature request to show that an external type parameter supports ref structs in tooltips about a type+method signature.

Inspiration taken from https://github.com/fsharp/fslang-design/pull/787 .
Will build upon the background impl of supporting `allows ref struct` typars from https://github.com/dotnet/fsharp/pull/17597  .
