Create `Packages::Nuget::Symbol` model and database table

Proposal

Create the Packages::Nuget::Symbol model and the corresponding database table in order to support storing symbols for a NuGet package. The Symbol model should have a reference to the package_packages table, enabling a NuGet package to have multiple symbol files associated with it. Furthermore, the Symbol model should be linked to a file on the object storage system, which will serve as the downloadable symbol file (.pdb).