The underlying SSL_get_negotiated_group function has been backported
into AWS-LC [1], so expose the feature for users of this TLS stack
as well. Note that even though it was actually added in AWS-LC 1.56.0,
we require AWSLC_API_VERSION >= 35 which was released in AWS-LC 1.57.0,
because API version wasn't incremented after this change. As the delta
is one minor version (less than two weeks), I consider this acceptable
to avoid relying on a proxy constant like TLSEXT_nid_unknown which
might be removed at some point.
[1] d6a37244ad
1d46c08689
