[PAC][libunwind] Move signing schemas which are part of public ABI to ptrauth.h header

#143230 contains many changes introducing different signing schemas for both internal data and things which are part of public ABI.

Signing schemas for the latter should be exposed in ptrauth.h header available for users.

See original comments:

1. https://github.com/llvm/llvm-project/pull/143230#discussion_r2155288616
2. https://github.com/llvm/llvm-project/pull/143230#discussion_r2155289051

And see also a proposal for adopting `static_assert`s for checking validity of discriminators: https://github.com/llvm/llvm-project/pull/143230#discussion_r2242166524

Read PR #143230 and the linked review discussions first, then identify which signing schemas describe public ABI rather than internal data. Move the public schemas into the ptrauth.h header and review the linked static_assert proposal for discriminator validity; done means users can access the public ABI schemas there.

Written by the indexing model from the issue text.