Add dispute_period and random_seed to SessionInfo

[rphmeier]

This will let us refactor the rolling session window based on dispute_period of the highest session, and change its uses in approval voting and the dispute coordinator to avoid hardcoded window sizes.

Alternatively, expose dispute_period by some other runtime API. However, the cleanest would be to use SessionInfo.

cc @ordian

[ordian]

We could also add random_seed from initializer to SessionInfo, that would be useful #3270.

[rphmeier]

I also recommend a new type for SessionInfo within the session_info runtime module so that changes in primitives don't silently invalidate runtime storage. I expect an Into<primitives::SessionInfo> for runtime::session_info::SessionInfo> would make the implementation of the runtime API easy.

We also have to account for the changing of the return type of the runtime API. This should be done with runtime API versioning and the #[changed_in] attribute as documented here