Support sub-1 sat/vB fee rates

[storopoli]

Describe the feature

We need to support sub 1 sats/vB in estimate_smart_fee RPC call.

Additional context

We set the minimum fee to be 1 sat/vB which WAS the correct setting for the Bitcoin network, but recently (July 2026) we had the minimum relay fee rate modified to accept relaying of transactions below 1 sat vB (see delving post for context, PRs, argumentation etc).

We are using sats/vB as u64. So it's either 0 or 1, we cannot have 0 < fee_rates < 1. To do this will need to change the API and will be a breaking change.