Add md_get_pdb_id API for reading PDB identity from #Pdb stream

[AaronRobinsonMSFT]

Add a new public API, md_get_pdb_id(), guarded by DNMD_PORTABLE_PDB, that extracts the 20-byte PDB ID (GUID + stamp) from the #Pdb stream of a portable PDB file.

This enables consumers to validate that a PDB file matches a PE assembly by comparing the PDB ID against the CodeView debug directory entry in the PE.

API:

#ifdef DNMD_PORTABLE_PDB
bool md_get_pdb_id(mdhandle_t handle, size_t* pdb_id_len, uint8_t* pdb_id);
#endif

Parameters:

• handle — metadata handle for a portable PDB
• pdb_id_len — in/out buffer size (must be ≥ 20)
• pdb_id — output buffer for the 20-byte PDB ID

Returns false if the handle does not contain a #Pdb stream.

[AaronRobinsonMSFT]

@jkoritzinsky I'm going to also push this through. Please let me know if you dislike the API shape. I'm fine changing it if you have another opinion on the data/shape.