[pallet_contracts] Add support for transient storage in contracts host functions (#4566)

Introduce transient storage, which behaves identically to regular
storage but is kept only in memory and discarded after every
transaction. This functionality is similar to the `TSTORE` and `TLOAD`
operations used in Ethereum.

The following new host functions have been introduced:
`get_transient_storage`
`set_transient_storage`
`take_transient_storage`
`clear_transient_storage`
`contains_transient_storage`
Note: These functions are declared as `unstable` and thus are not
activated.

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Co-authored-by: command-bot <>
Co-authored-by: PG Herveou <pgherveou@gmail.com>
Co-authored-by: Alexander Theißen <alex.theissen@me.com>