Normalize `ProtoSchema` descriptor

[haphungw]

BigQuery Storage Write API requires proto_descriptor in ProtoSchema to be self-contained with no external unresolved imports.

1. Port Go's NormalizeDescriptor (managedwriter/adapt/protoconversion.go:538).
2. Flatten all nested type dependencies into the primary DescriptorProto.
3. Strip external file dependencies so the descriptor is accepted by the Storage Write API backend without unresolvable dependency errors.