Introduce a minimum buy amount

Introduces a minimum buy Bitcoin amount similar to the maximum amount already present.
For the CLI the minimum amount is enforced by waiting until at least the minimum is available as max-giveable amount.

swap/src/protocol/alice/behaviour.rs

@@ -84,6 +84,7 @@ where
     pub fn new(
         balance: monero::Amount,
         lock_fee: monero::Amount,
+        min_buy: bitcoin::Amount,
         max_buy: bitcoin::Amount,
         latest_rate: LR,
         resume_only: bool,
@@ -93,6 +94,7 @@ where
             spot_price: spot_price::Behaviour::new(
                 balance,
                 lock_fee,
+                min_buy,
                 max_buy,
                 latest_rate,
                 resume_only,