smite-ir: implement ChannelUpdateGenerator

[devvaansh]

Generates programs that build and send correctly-signed channel_update messages to the target.

The signing key, chain hash, and short channel id are drawn via pick_variable so they can be shared with other gossip messages (e.g. a preceding channel_announcement) when generators are composed in the future.

message_flags and channel_flags are exposed as plain U8 variables so that OperationParamMutator can flip individual bits, including must_be_one, direction, and disable.

Note: fee_base_msat and fee_proportional_millionths are currently typed as Amount until the ForwardingFee variable type lands in #99.

Depends on #110 (BuildChannelUpdate operation) and #99 (ForwardingFee variable type).

Ref: #71