The module should support AES gcm mode.

[rcosnita]

At the moment the module does not support gcm mode. In order to keep backward compatibility, I suggest we introduce a directive which allows us to switch between cbc and gcm mode. By default, CBC should be enabled in order to keep backward compatibility.

encrypted_session_key $key; # introduced in PR https://github.com/openresty/encrypted-session-nginx-module/pull/24
encrypted_session_iv $iv; # introduced in PR https://github.com/openresty/encrypted-session-nginx-module/pull/24
encrypted_session_iv_in_content; # introduced in PR https://github.com/openresty/encrypted-session-nginx-module/pull/24
encrypted_session_mode gcm; # possible values are gcm or cbc.