* Move `trait ParachainHost` to a separate version independent module

`trait ParachainHost` is no longer part of a specific primitives
version. Instead there is a single trait for stable and staging api
versions. The trait contains stable AND staging methods. The latter are
explicitly marked as unstable.

* Fix `use` primitives

`polkadot_primitives::v2` becomes `polkadot_primitives::runtime_api`

* Staging API declaration and stubs

Introduces the concept for 'staging functions' in runtime API. These
functions are still in testing and they are meant to be used only
within test networks (Westend).
They coexist with the stable calls for technical reasons - maintaining
different runtime APIs for different networks is hard to implement.

Check the doc comments in source files for more details how the staging
API should be used.

* Add new staging method - get_session_disputes()

Add `staging_get_session_disputes` to `ParachainHost` as the first
method of the staging API.

* Hide vstaging runtime api implementations  behind feature flag

* Fix test runtime

* fn staging_get_session_disputes() is renamed to fn staging_get_disputes()

* add epoch index to well known keys

* fmt

* allow converting payloads _up_

* convert to superpayload in statement-distribution

* Update primitives/src/v2/signed.rs

Co-authored-by: Andronik <[email protected]>

Co-authored-by: Andronik <[email protected]>

* add babe randomness storage keys to well known keys

* Update primitives/src/v1/mod.rs

Co-authored-by: Bastian Köcher <[email protected]>

* remove v0 primitives from polkadot-primitives

* first pass: remove v0

* fix fallout in erasure-coding

* remove v1 primitives, consolidate to v2

* the great import update

* update runtime_api_impl_v1 to v2 as well

* guide: add `Version` request for runtime API

* add version query to runtime API

* reintroduce OldV1SessionInfo in a limited way

This is the first step to close
https://github.com/paritytech/polkadot/issues/4524

* session_info: v2 + migration

* use primitives::v2

* use polkadot_primitives::v2

* impl primitives::v2

* fix approval-voting tests

* fix other tests

* hook storage migration up

* backwards compat (1)

* backwards compat (2)

* fmt

* fix tests

* FMT

* do not reexport v1 in v2

* fmt

* set storage version to 1

Co-authored-by: Javier Viola <[email protected]>