rest: unify approach to supporting multiple interactive session types

[audrium]

Some parts of the r-w-controller can handle multiple session types, while other parts assume that all sessions are Jupyter notebooks.

For example:

• While opening the session we have interactive_session_type as a parameter which can in theory handle different session types
• While closing the session we just assume it's a notebook

We should have a unified approach to supporting multiple interactive session types. Session closure endpoint should also have interactive_session_type as a parameter. Session type could be added as a pod label for better filtering which would make it easier to find and manage sessions of a particular type. Please note that reana-admin interactive_session_cleanup CLI command might also be improved depending on the implementation of this issue.

_Originally posted by @mdonadoni in reanahub/reana-server#577 (comment)