Test external rust bindings build with additional libcryptos

[goatgoose]

Problem:

#5229 added unit test coverage for the external build Rust bindings feature by building s2n-tls externally with AWS-LC. However, it's possible for s2n-tls to be built with an arbitrary libcrypto and provided to the Rust bindings with the external build feature, which there isn't currently test coverage for.

Solution:

We should test the external build feature with additional libcryptos, minimally OpenSSL 1.1.1.

In implementing #5229, it was discovered that some unit tests assume that AWS-LC is the linked libcrypto:

s2n-tls/bindings/rust/extended/s2n-tls/src/testing/s2n_tls.rs

Lines 51 to 52 in 0ece10f

	pair.handshake().unwrap();
	assert_eq!(pair.client.kem_group_name(), Some("X25519MLKEM768"));

These tests should be updated to avoid this assumption.

[goatgoose]

Alternative solution: if there isn't a use case for other external libcryptos we could just update our documentation to only support AWS-LC.