Update API when we add groups other than EC to TLS1.3

[maddeleine]

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Problem:

Currently, the PR #2908 relies on the fact that we only do ECC in TLS1.3. If we ever add more than ECC to TLS1.3, the function s2n_connection_get_curve will no longer work.

Solution:

When we add a new group to s2n-tls, we should update the function s2n_connection_get_curve as well to make sure it is still accurate.

• Does this change what S2N sends over the wire? no
• Does this change any public APIs? Eh, it adds functionality to a current API
• Which versions of TLS will this impact?
  versions TLS13 and up

Requirements / Acceptance Criteria:

s2n_connection_get_curve is updated when a new group is added.

• RFC links: N/A
• Related Issues: N/A
• Will the Usage Guide or other documentation need to be updated? Nope
• Testing: Unit tests are fine
  • Will this change trigger SAW changes? N/A
  • Should this change be fuzz tested? N/A

Out of scope:

N/A