[FEAT]: Move file /internal/session/interface.go from folder /internal

[Kalinin-Andrey]

Is your feature request related to a problem? Please describe.

This small change will allow to:

1. create implementation of SessionLoader for various client-server solutions;
2. will allow to make a multi-user application, because. it will be possible for each user to load his own session.

Describe the solution you'd like

Just move file /internal/session/interface.go from folder /internal to make types SessionLoader and Session public.

Thank you!