Interop Gen: Add configuration option for including non-exported declarations

[nikeokoronkwo]

For a good number of libraries, we may want to generate interop types for declarations even though they are not explicitly exported (maybe we know they are exported by default, in some other way, or they are in the global scope, or the user handles the exporting process him/herself)

This issue is to add support for such a configuration option to allow generating declarations for all declarations, including those not exported