openpgp: Port the OpenSSL backend to ossl.

Neal H. Walfieldrequested to merge
neal/post-to-ossl into main

  • Port the OpenSSL backend from using the rust-openssl bindings to the ossl bindings. The latter use the new OpenSSL 3 interfaces instead of the deprecated legacy ones.

    • Open issues:

      • Fewer supported algorithms: No DSA, Brainpool, 3DES, Camellia, Blowfish, Cast5, IDEA.

      • Currently leaks secrets into the heap.

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