Expose scheduling group API to Rust

[patjed41]

We would like to expose a simplified version of the API implemented in scheduling.hh to Rust. More specifically, we want to expose:

• seastar::schedoluing_group with all its methods except get_specific,
• functions: create_scheduling_group, destroy_scheduling_group, rename_scheduling_group.
• functions: max_scheduling_groups, current_scheduling_group, default_scheduling_group.

Additionally, we want to use exposed seastar::schedoluing_group to finish implementation of timer's specializations (see #32).

We don't want to expose the rest of the scheduling API for now because it might be really hard to implement and is not that important for us.