Refactor polkadot-parachain service for more code reuse (#3511)

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.

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Co-authored-by: Kian Paimani <[email protected]>
Co-authored-by: Francisco Aguirre <[email protected]>
Co-authored-by: Adrian Catangiu <[email protected]>
Co-authored-by: Egor_P <[email protected]>
Co-authored-by: Dmitry Markin <[email protected]>
Co-authored-by: Xiliang Chen <[email protected]>
Co-authored-by: Andrei Eres <[email protected]>
Co-authored-by: Alexander Samusev <[email protected]>
Co-authored-by: Alin Dima <[email protected]>
Co-authored-by: gupnik <[email protected]>
Co-authored-by: Liam Aharon <[email protected]>
Co-authored-by: Dónal Murray <[email protected]>