feat: Add formatting method for routing header values
feat: Add formatting method for routing header values

This will then be called by the generator for both implicit and explicit headers that are not for string fields.

It's slightly annoying to have to include a type name when calling
the method, but it's only ever used in autogenerated code, and I
think that beats exposing it in a less specific context.

Release-As: 4.2.0-alpha01

googleapis/gax-dotnet,jskeet/gax-dotnet,jskeet/gax-dotnet,googleapis/gax-dotnet