This is a refactoring, no changes to the logic are included (if you find
some, report :D).

## Change Overview
In https://github.com/paritytech/polkadot-sdk/issues/2455, dependency on
actual runtimes was removed for the system parachains. This means that
the trait bounds we have on various `start_node_xy` do not check
anything anymore, since they all point to the same runtime. Exception is
asset-hub-polkadot which uses a different key type.

This PR unifies the different nodes as much as possible.
`start_node_impl` is doing the heavy lifting and has been made a bit
more flexible to support the rpc extension instantiation that was there
before.

The generics for `Runtime` and `AuraId` have been removed where
possible. The fake runtime is imported as `FakeRuntime` to make it very
clear to readers that it is not the generic parameter.

Multiple nodes where using the same import queue/start_consensus
closure, they have been unified.

---------

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>
Co-authored-by: Adrian Catangiu <adrian@parity.io>
Co-authored-by: Egor_P <egor@parity.io>
Co-authored-by: Dmitry Markin <dmitry@markin.tech>
Co-authored-by: Xiliang Chen <xlchen1291@gmail.com>
Co-authored-by: Andrei Eres <eresav@me.com>
Co-authored-by: Alexander Samusev <41779041+alvicsam@users.noreply.github.com>
Co-authored-by: Alin Dima <alin@parity.io>
Co-authored-by: gupnik <nikhilgupta.iitk@gmail.com>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>
Co-authored-by: Dónal Murray <donalm@seadanda.dev>