Add rudimentary database watcher

Adds a simple database watcher. At this point it's just one process, but
the plan is to allow different implementations that inform the local running
workers of changes that have occured on entities of interest in the database.

Signed-off-by: Gabriel Adrian Samfira <gsamfira@cloudbasesolutions.com>

internal/testing/mock_watcher.go

@@ -0,0 +1,45 @@
+//go:build testing
+// +build testing
+
+package testing
+
+import "github.com/cloudbase/garm/database/common"
+
+type MockWatcher struct{}
+
+func (w *MockWatcher) RegisterProducer(_ string) (common.Producer, error) {
+	return &MockProducer{}, nil
+}
+
+func (w *MockWatcher) RegisterConsumer(_ string, _ ...common.PayloadFilterFunc) (common.Consumer, error) {
+	return &MockConsumer{}, nil
+}
+
+type MockProducer struct{}
+
+func (p *MockProducer) Notify(_ common.ChangePayload) error {
+	return nil
+}
+
+func (p *MockProducer) IsClosed() bool {
+	return false
+}
+
+func (p *MockProducer) Close() {
+}
+
+type MockConsumer struct{}
+
+func (c *MockConsumer) Watch() <-chan common.ChangePayload {
+	return nil
+}
+
+func (c *MockConsumer) SetFilters(_ ...common.PayloadFilterFunc) {
+}
+
+func (c *MockConsumer) Close() {
+}
+
+func (c *MockConsumer) IsClosed() bool {
+	return false
+}