The runtime code of a parachain can be replaced on the relay-chain via:

[cumulus]:
[enact_authorized_upgrade](https://github.com/paritytech/polkadot-sdk/blob/1a38d6d6/cumulus/pallets/parachain-system/src/lib.rs#L661);
this is used for a runtime upgrade when a parachain is not bricked.

[polkadot] (these are used when a parachain is bricked):
-
[force_set_current_code](https://github.com/paritytech/polkadot-sdk/blob/1a38d6d6/polkadot/runtime/parachains/src/paras/mod.rs#L823):
immediately changes the runtime code of a given para without a pvf check
(root).
-
[force_schedule_code_upgrade](https://github.com/paritytech/polkadot-sdk/blob/1a38d6d6/polkadot/runtime/parachains/src/paras/mod.rs#L864):
schedules a change to the runtime code of a given para including a pvf
check of the new code (root).
-
[schedule_code_upgrade](https://github.com/paritytech/polkadot-sdk/blob/1a38d6d6/polkadot/runtime/common/src/paras_registrar.rs#L395):
schedules a change to the runtime code of a given para including a pvf
check of the new code. Besides root, the parachain or parachain manager
can call this extrinsic given that the parachain is unlocked.

Polkadot signals a parachain to be ready for a runtime upgrade through
the
[GoAhead](https://github.com/paritytech/polkadot-sdk/blob/e4949344

/polkadot/primitives/src/v5/mod.rs#L1229)
signal.

When in cumulus `enact_authorized_upgrade` is executed, the same
underlying helper function of `force_schedule_code_upgrade` &
`schedule_code_upgrade`:
[schedule_code_upgrade](https://github.com/paritytech/polkadot/blob/09b61286da11921a3dda0a8e4015ceb9ef9cffca/runtime/parachains/src/paras/mod.rs#L1778),
is called on the relay-chain, which sets the `GoAhead` signal (if the
pvf is accepted).

If Cumulus receives the `GoAhead` signal from polkadot without having
the `PendingValidationCode` ready, it will panic
([ref](https://github.com/paritytech/polkadot/pull/7412)). For
`enact_authorized_upgrade` we know for sure the `PendingValidationCode`
is set. On the contrary, for `force_schedule_code_upgrade` &
`schedule_code_upgrade` this is not the case.

This PR includes a flag such that the `GoAhead` signal will only be set
when a runtime upgrade is enacted by the parachain
(`enact_authorized_upgrade`).

additional info: https://github.com/paritytech/polkadot/pull/7412

Closes #641

---------

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