lnwallet+sweep: introduce TxPublisher to handle fee bump

This commit adds `TxPublisher` which implements `Bumper` interface. This
is part one of the implementation that focuses on implementing the
`Broadcast` method which guarantees a tx can be published with
RBF-compliant. It does so by leveraging the `testmempoolaccept` API,
keep increasing the fee rate until an RBF-compliant tx is made and
broadcasts it.

This tx will then be monitored by the `TxPublisher` and in the following
commit, the monitoring process will be added.

chainntnfs/mocks.go

@@ -1,6 +1,7 @@
 package chainntnfs
 
 import (
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightningnetwork/lnd/fn"
 	"github.com/stretchr/testify/mock"
@@ -50,3 +51,73 @@ func (m *MockMempoolWatcher) LookupInputMempoolSpend(
 
 	return args.Get(0).(fn.Option[wire.MsgTx])
 }
+
+// MockNotifier is a mock implementation of the ChainNotifier interface.
+type MockChainNotifier struct {
+	mock.Mock
+}
+
+// Compile-time check to ensure MockChainNotifier implements ChainNotifier.
+var _ ChainNotifier = (*MockChainNotifier)(nil)
+
+// RegisterConfirmationsNtfn registers an intent to be notified once txid
+// reaches numConfs confirmations.
+func (m *MockChainNotifier) RegisterConfirmationsNtfn(txid *chainhash.Hash,
+	pkScript []byte, numConfs, heightHint uint32,
+	opts ...NotifierOption) (*ConfirmationEvent, error) {
+
+	args := m.Called(txid, pkScript, numConfs, heightHint)
+	if args.Get(0) == nil {
+		return nil, args.Error(1)
+	}
+
+	return args.Get(0).(*ConfirmationEvent), args.Error(1)
+}
+
+// RegisterSpendNtfn registers an intent to be notified once the target
+// outpoint is successfully spent within a transaction.
+func (m *MockChainNotifier) RegisterSpendNtfn(outpoint *wire.OutPoint,
+	pkScript []byte, heightHint uint32) (*SpendEvent, error) {
+
+	args := m.Called(outpoint, pkScript, heightHint)
+	if args.Get(0) == nil {
+		return nil, args.Error(1)
+	}
+
+	return args.Get(0).(*SpendEvent), args.Error(1)
+}
+
+// RegisterBlockEpochNtfn registers an intent to be notified of each new block
+// connected to the tip of the main chain.
+func (m *MockChainNotifier) RegisterBlockEpochNtfn(epoch *BlockEpoch) (
+	*BlockEpochEvent, error) {
+
+	args := m.Called(epoch)
+	if args.Get(0) == nil {
+		return nil, args.Error(1)
+	}
+
+	return args.Get(0).(*BlockEpochEvent), args.Error(1)
+}
+
+// Start the ChainNotifier. Once started, the implementation should be ready,
+// and able to receive notification registrations from clients.
+func (m *MockChainNotifier) Start() error {
+	args := m.Called()
+
+	return args.Error(0)
+}
+
+// Started returns true if this instance has been started, and false otherwise.
+func (m *MockChainNotifier) Started() bool {
+	args := m.Called()
+
+	return args.Bool(0)
+}
+
+// Stops the concrete ChainNotifier.
+func (m *MockChainNotifier) Stop() error {
+	args := m.Called()
+
+	return args.Error(0)
+}