[Feature request] Add method_pref session function

[pkittenis]

From mail group

Add method_pref function to session for libssh2_session_method_pref.

Also related function libssh2_session_methods.

[Red-M]

I had this in #90

[pkittenis]

If you'd like to make a PR I'm up for reviewing it. The contributing guide has detailed information on how to generate cython code, run tests and make PRs.

Tests passing is a requirement for any PR to be merged.