sweep: introduce BudgetInputSet to manage budget-based inputs

This commit adds `BudgetInputSet` which implements `InputSet`. It
handles the pending inputs based on the supplied budgets and will be
used in the following commit.

sweep/sweeper.go

@@ -52,11 +52,22 @@ type Params struct {
 
 	// Force indicates whether the input should be swept regardless of
 	// whether it is economical to do so.
+	//
+	// TODO(yy): Remove this param once deadline based sweeping is in place.
 	Force bool
 
 	// ExclusiveGroup is an identifier that, if set, prevents other inputs
 	// with the same identifier from being batched together.
 	ExclusiveGroup *uint64
+
+	// DeadlineHeight specifies an absolute block height that this input
+	// should be confirmed by. This value is used by the fee bumper to
+	// decide its urgency and adjust its feerate used.
+	DeadlineHeight fn.Option[int32]
+
+	// Budget specifies the maximum amount of satoshis that can be spent on
+	// fees for this sweep.
+	Budget btcutil.Amount
 }
 
 // ParamsUpdate contains a new set of parameters to update a pending sweep with.
@@ -196,6 +207,11 @@ type pendingInput struct {
 	rbf fn.Option[RBFInfo]
 }
 
+// String returns a human readable interpretation of the pending input.
+func (p *pendingInput) String() string {
+	return fmt.Sprintf("%v (%v)", p.Input.OutPoint(), p.Input.WitnessType())
+}
+
 // parameters returns the sweep parameters for this input.
 //
 // NOTE: Part of the txInput interface.