refactor: use feature probe for AEAD gate logic instead of AWS-LC/BoringSSL macros

[jouho]

Resolved issues:

Resolves #4575

Description of changes:

• simplifies the gating logic for AEAD support by using S2N_LIBCRYPTO_SUPPORTS_EVP_AEAD_TLS

Call-outs:

I also tried to update this macro:

s2n-tls/crypto/s2n_cipher.h

Lines 29 to 31 in 03f9301

	#if defined(OPENSSL_IS_BORINGSSL) || defined(OPENSSL_IS_AWSLC)
	#define S2N_CIPHER_AEAD_API_AVAILABLE
	#endif

But there is a case when S2N_CHACHA20_POLY1305_AVAILABLE_BSSL_AWSLC is on, and S2N_LIBCRYPTO_SUPPORTS_EVP_AEAD_TLS is off (failed job: S2NGeneralBatch, s2nUnitCRT).

s2n-tls/crypto/s2n_aead_cipher_chacha20_poly1305.c

Lines 31 to 32 in 03f9301

	#if defined(OPENSSL_IS_BORINGSSL) || defined(OPENSSL_IS_AWSLC)
	#define S2N_CHACHA20_POLY1305_AVAILABLE_BSSL_AWSLC

This causes compile error because when S2N_CHACHA20_POLY1305_AVAILABLE_BSSL_AWSLC is on, it uses key->evp_aead_ctx to set encryption key:

s2n-tls/crypto/s2n_aead_cipher_chacha20_poly1305.c

Line 201 in 03f9301

POSIX_GUARD_OSSL(EVP_AEAD_CTX_init(key->evp_aead_ctx, EVP_aead_chacha20_poly1305(), in->data, in->size, S2N_TLS_CHACHA20_POLY1305_TAG_LEN, NULL), S2N_ERR_KEY_INIT);

But key->evp_aead_ctx is not defined because S2N_LIBCRYPTO_SUPPORTS_EVP_AEAD_TLS is off:

s2n-tls/crypto/s2n_cipher.h

Lines 33 to 38 in 03f9301

	struct s2n_session_key {
	EVP_CIPHER_CTX *evp_cipher_ctx;
	#if defined(S2N_CIPHER_AEAD_API_AVAILABLE)
	EVP_AEAD_CTX *evp_aead_ctx;
	#endif
	};

Not exactly sure if S2N_LIBCRYPTO_SUPPORTS_EVP_AEAD_TLS would imply S2N_CHACHA20_POLY1305_AVAILABLE_BSSL_AWSLC I assume it doesn't since non-BSSL or non-AWSLC could enable AEAD? but I could be wrong and wanted to double check. If it does, we can also use S2N_LIBCRYPTO_SUPPORTS_EVP_AEAD_TLS to define S2N_CHACHA20_POLY1305_AVAILABLE_BSSL_AWSLC macro, and S2N_CIPHER_AEAD_API_AVAILABLE could potentially also be defined with S2N_LIBCRYPTO_SUPPORTS_EVP_AEAD_TLS for further cleanup

Testing:

Confirmed all unit tests pass. Will address any CI failures.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.