pallet-contracts: Rent projection RPC (#4754)

* Initial approach

* Introduce the pallet-contracts-common crate

* Add rent::compute_rent_projection

* Wire everything together

* Fix build error.

* Rename EvictionDate → EvictionAt.

* Clean.

* Renaming and cleaning.

* Add documentation for rent_projection RPC.

* Add documentation for rent_projection runtime API.

* Refactor rent_budget.

Merge it with subsistence_treshold.

* Bump impl_version

* Constrain RPC impl with Block::Header::Number.

* Rename pallet-contracts-common into -primitives

* Add a comment for `compute_rent_projection` on the usage

* Small tidying