Allow `ApprovalController` requests to be updated

[FrederikBolding]

In case of more dynamic approvals, that fetch data before showing it to the user, we can allow for updates requestData. This will allow for example the SnapController to show a prompt for snap installation with a spinner until the data has been fetched to populate the prompt.

We don't necessarily need to be able to update the entire approval, we could restrict it to only allow updating requestData.

[Gudahtt]

Perhaps we could keep the mutable data associated with an approval separate from the initial request? i.e. leave requestData alone, but add a mutable requestState for arbitrary state associated with a specific approval request.

That would fulfill this need while keeping the normal reuqestData immutable and simpler to understand.

[FrederikBolding]

I'm fine with this! @GuillaumeRx do you mind making this change in your PR?