Add .replaceDevicePrivateKey() value to PacketTunnelActor's Command enum

[acb-mv]

This is currently a no-op and nothing sends it, but it is there for passing the key to the actor.

Also, Command is now PacketTunnelActor.Command. This is less ambiguous and polluting of namespace than having a globally scoped type named Command that serves only to couple two components; the other alternative was naming it PacketTunnelActorCommand, but for the cost of one character, the prefix disappears when in scope.

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This change is 

[linear]

IOS-552 Add PacketTunnelActor Command value for passing negotiated PQ key

[buggmagnet]

We just need to change the type for the new command introduced, but looks good otherwise.

Reviewed 1 of 5 files at r1, 2 of 4 files at r2, all commit messages.
Reviewable status: 3 of 5 files reviewed, 1 unresolved discussion (waiting on @acb-mv)

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ios/PacketTunnelCore/Actor/PacketTunnelActorCommand.swift line 40 at r2 (raw file):

        /// Update the device private key, as per post-quantum protocols
        case replaceDevicePrivateKey(PrivateKey)

The type should be a SharedKey here (it already exists in WireguardKitTypes)

[rablador]

Reviewed 1 of 5 files at r1, 3 of 4 files at r2, 1 of 1 files at r3, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @acb-mv)

[buggmagnet]

Reviewed 2 of 4 files at r2, 1 of 1 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved