Expose the EVP_AEAD Interface

Right now, the [`EVP_AEAD` Interface](https://commondatastorage.googleapis.com/chromium-boringssl-docs/aead.h.html) is the only way to access modern primitives like AES-GCM-SIV and XChaCha20-Poly1305. Exposing it would make it easier for folks to use these primitives in their rust programs.

For use in a different project, I wrote a [quick patch](https://github.com/sgmenda/boring/commit/49d046b0892b95845a1d15de506fd487fe368d46) that exposes `EVP_AEAD` in a way heavily inspired by how [`symm.rs`](https://github.com/cloudflare/boring/blob/c4e1f474973744b32579797c1ff51335002a4491/boring/src/symm.rs) currently exposes `EVP_CIPHER`.

If you are interested in this, I can clean up my patch (write more examples, come up with a better name than `AeadCrypter`,...) and submit a PR.