Add flags parameter to LIBSSH2_AUTHAGENT_SIGN_FUNC

[MichaelBuckley]

Another change from the Panic repo. Allows agent signing using rsa-sha2-256 and rsa-sha-512 when setting a callback with LIBSSH2_CALLBACK_AUTHAGENT_SIGN.

[vszakats]

Thanks!

Wouldn't this be an ABI breaking change? In such case perhaps we might do a libssh2_session_callback_set3, while also fixing up other stuff like going with size_t, if it makes sense here?

[MichaelBuckley]

It's definitely an ABI breaking change. In this case, I just created a pull request from a commit in our repo that someone else at Panic wrote, without considering if it broke ABI.

If we wanted to preserve ABI, rather than making a libssh2_session_callback_set3, I think we'd have to create a LIBSSH2_CALLBACK_AUTHAGENT_SIGN_2 and a LIBSSH2_AUTHAGENT_SIGN_2. The callback registration wouldn't need to change, since how users interact with this function is by calling LIBSSH2_AUTHAGENT_SIGN.

[vszakats]

If we wanted to preserve ABI, rather than making a libssh2_session_callback_set3, I think we'd have to create a LIBSSH2_CALLBACK_AUTHAGENT_SIGN_2 and a LIBSSH2_AUTHAGENT_SIGN_2. The callback registration wouldn't need to change, since how users interact with this function is by calling LIBSSH2_AUTHAGENT_SIGN.

You're right, that's a much better way to go.