bindings(s2n-tls): add kem_group api

[jmayclin]

Problem:

The kem_group api is not available in the rust bindings.

s2n-tls/api/s2n.h

Lines 3311 to 3317 in 2ba3dda

	/**
	* Function to get the human readable KEM group name for the connection.
	*
	* @param conn A pointer to the s2n connection
	* @returns A human readable string for the KEM group. If the connection is < TLS1.3 or there is no KEM group configured returns "NONE"
	*/
	S2N_API extern const char *s2n_connection_get_kem_group_name(struct s2n_connection *conn);

Solution:

This should be added to the rust bindings.

Requirements / Acceptance Criteria:

• the kem_group api should be added to the s2n-tls crate
• documentation should make the differences between kem_group and kem_name clear.
• the pq_sanity_check in integration/src/libs.rs should be updated to use the default_pq policy and assert on the negotiated kem group
• the pq_handshake in integration/src/network/tls_client.rs should be updated to use the default_pq policy and assert on the negotiated kem group.