Adds new event phase `Initialization` (#5302)

* Adds new event phase `Initialization`

Every event that was deposited inside of `on_initialize` was assigned to
the `ApplyExtrinsic(0)` phase which wasn't correct. This pr introduces a
new phase `Initialization`. This is the new phase while initializing
a block. After initialization we switch to `ApplyExtrinsic(N)` and at
the end to `Finalization` as before.

* Set `ExecutionPhase` in `initialize`

* Increment `spec_version`