To create non-BTC offers requires a plugin which implements the "currencyconvert" API. An example is provided by https://github.com/lightningd/plugins/tree/master/currencyrate but it hasn't been updated in a while.

We should create an in-tree one.

1. Rust, not Python.
2. Check currency sources still work (is there a Blockstream one we can use?)
3. If node is configured to use a proxy, MUST use the proxy (don't reveal Tor nodes)
4. Maybe be more intelligent with multiple sources: refresh one at a time per hour, and if that reveals significant difference, go for more?

We need this in tree to support a new "recurringpay" command API to handle recurring BOLT12 offers automatically (which are usually in non-sat demoninations).