sweep: introduce Bumper interface to handle RBF

This commit adds a new interface, `Bumper`, to handle RBF for a given
input set. It's responsible for creating the sweeping tx using the input
set, and monitors its confirmation status to decide whether a RBF should
be attempted or not.

We leave implementation details to future commits, and focus on mounting
this `Bumper` interface to our sweeper in this commit.

sweep/fee_bumper_test.go

@@ -0,0 +1,52 @@
+package sweep
+
+import (
+	"testing"
+
+	"github.com/btcsuite/btcd/wire"
+	"github.com/stretchr/testify/require"
+)
+
+// TestBumpResultValidate tests the validate method of the BumpResult struct.
+func TestBumpResultValidate(t *testing.T) {
+	t.Parallel()
+
+	// An empty result will give an error.
+	b := BumpResult{}
+	require.ErrorIs(t, b.Validate(), ErrInvalidBumpResult)
+
+	// Unknown event type will give an error.
+	b = BumpResult{
+		Tx:    &wire.MsgTx{},
+		Event: sentinalEvent,
+	}
+	require.ErrorIs(t, b.Validate(), ErrInvalidBumpResult)
+
+	// A replacing event without a new tx will give an error.
+	b = BumpResult{
+		Tx:    &wire.MsgTx{},
+		Event: TxReplaced,
+	}
+	require.ErrorIs(t, b.Validate(), ErrInvalidBumpResult)
+
+	// A failed event without a failure reason will give an error.
+	b = BumpResult{
+		Tx:    &wire.MsgTx{},
+		Event: TxFailed,
+	}
+	require.ErrorIs(t, b.Validate(), ErrInvalidBumpResult)
+
+	// A confirmed event without fee info will give an error.
+	b = BumpResult{
+		Tx:    &wire.MsgTx{},
+		Event: TxConfirmed,
+	}
+	require.ErrorIs(t, b.Validate(), ErrInvalidBumpResult)
+
+	// Test a valid result.
+	b = BumpResult{
+		Tx:    &wire.MsgTx{},
+		Event: TxPublished,
+	}
+	require.NoError(t, b.Validate())
+}