Add support for Portable PDB interface

See [`ISymUnmanagedReader`](https://learn.microsoft.com/dotnet/framework/unmanaged-api/diagnostics/isymunmanagedreader-interface).

This should also include deprecating https://github.com/AaronRobinsonMSFT/PPDB.

The issue references the ISymUnmanagedReader interface for Portable PDB support. Start by examining the existing metadata APIs in the repository to understand the current PDB handling. Look for where symbol reading interfaces are implemented and see how the deprecated PPDB project relates. Determine what new methods or classes need to be added to support the interface, and check for any tests related to debugging or symbols to verify the implementation.

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