Add API to get the s2n_mode used when calling s2n_connection_new

[jeking3]

Problem:

Once one calls s2n_connection_new with a s2n_mode, there is no way to retrieve the s2n_mode of the connection.

Solution:

Provide an API, for example s2n_connection_get_mode, to retrieve the mode.

• Does this change what S2N sends over the wire? No
• Does this change any public APIs? Yes. See above.
• Which versions of TLS will this impact? N/A

Requirements / Acceptance Criteria:

One is able to get the s2n_mode that was used with s2n_connection_new.