Add back metadata specs files

[tmat]

#35095 removed files from src/System.Reflection.Metadata/specs. This change broke links to the specs used in other repos.
It'd be better to add the files back with a link to the new location.

[ericstj]

You shouldn't have links which rely on the position of a file in source control. We don't maintain that as a public contract. Can you update the external references, ideally to use fwlinks/aka.ms so that they're resilient to any changes in the future?

cc @jkotas @jnm2

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