Add initial support for MLKEM768 (without any new Security Policies)

[alexw91]

Resolved issues:

None

Note: PR #4823 should be reviewed and merged before this PR.

Description of changes:

Part 3 in a multi-part series to add X25519MLKEM768 support to s2n.

This PR includes the following changes:

• Add support for new send_kem_first boolean parameter for Hybrid PQ KeyShares (needed for X25519MLKEM768).
• Add support for 1 new KEM: MLKEM768
• Add support for 2 new Hybrid KemGroups: X25519MLKEM768 and SecP256r1MLKEM768
• Add new self-talk unit tests to negotiate X25519MLKEM768 and confirm shared secret calculation is correct.

Call-outs:

No new s2n-tls security policies with X25519MLKEM768 have been added (other than test_all Policy). These new MLKEM-768 security policies will be coming in a later PR.

Previous PR's must be merged first:

• Part 1: Update PQ code to be generic over EVP_KEM API's #4810
• Part 2: Add ML-KEM Feature Probe and Test #4823

Testing:

• s2n self-talk unit tests with X25519MLKEM768
• Unit tests confirm that s2n-tls hybrid shared secret calculation matches 2nd independent Python implementation.

Is this a refactor change? If so, how have you proved that the intended behavior hasn't changed?

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

[lrstewart]

How is this change related to the rest of this PR?

[alexw91]

It's not, this change is a cleanup that probably should have been included in the previous PR. This is the only other use of s2n_libcrypto_awslc_api_version() in the s2n unit tests. Now that this API was added to s2n_libcrypto.h I updated this test to include the header instead of declaring the API itself.