Custom expiry time for `addinvoice`

[kristapsk]

There is expiry in protobuf spec, but no way to set custom value for it in addinvoice call.

Recently LndHub support was added to SatSale payment processor. For CLN and LND backends expiry is set to configurable invoice expiry, which defaults to 3600 seconds. Would be useful to be able to do the same for LndHub, otherwise currently LN invoice expiry time is bigger than default invoice expiry time for SatSale, so there is higher possibility merchant will have to deal with returning payments.

[frpollg]

Any chance to set the expiration time when creating lightning invoices? Thx!