Add getUpgradeTimeout function to calculate timeout for packet flushing in upgrades

[damiannolan]

Summary

Determine how upgrade timeouts for packet processing will be calculated. E.g. via on-chain parameter?

As stated by the spec:

getUpgradeTimeout will return the upgrade timeout specified for the given channel. This may be a chain-wide parameter, or it can be a parameter chosen per channel. This is an implementation-level detail, so only the function signature is specified here. Note this should retrieve some stored timeout delta for the channel and add it to the current height and time to get the absolute timeout values.

function getUpgradeTimeout(portIdentifier: string, channelIdentifier: string) UpgradeTimeout {
}

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For Admin Use

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[damiannolan]

See #4441 for more context, should be linked to this issue