Add ChainID stamped invoices and CurrencyConversion by the intermediary

[NiloCK]

PR:

• enriches the Invoice struct with a ChainID (It's back!)
• adds a conversion function to convert chainA invoice to an equal valued chainB invoice
• a simple test for this function
• a hook for Irene to convert currencies during cross network payments

Workflow remains identical for Alice and Bob - Alice requests an invoice denominated in her local currency, which Bob provides, with a hashlock.

Irene converts the currency during the handoff between Alice:Irene and Irene:Bob channels.

Manually e2e tested w/ two local hardhat nodes with a specifically configured currency value difference.