Infer index provider topic from network name by default

Index provider integration uses a gossipsub topic to announce changes to
the advertised content. The topic name was fixed to the default topic
which is `/indexer/ingest/mainnet`.

In the case of lotus, the gossipsub validators enforce a list of topics
the instance is permitted to join by setting subscription filter option
when `PubSub` instance is constructed via DI.

Having the fixed topic name meant that any SP starting up a node on a
network other than `mainnet` would have to override the default config
to avoid the node crashing when index provider is enabled.

Instead of a fixed default, the changes here infer the allowed indexer
topic name from network name automatically if the topic configuration is
left empty.

Fixes #8510

node/config/def.go

@@ -199,8 +199,10 @@ func DefaultStorageMiner() *StorageMiner {
 			Enable:               true,
 			EntriesCacheCapacity: 1024,
 			EntriesChunkSize:     16384,
-			TopicName:            "/indexer/ingest/mainnet",
-			PurgeCacheOnStart:    false,
+			// The default empty TopicName means it is inferred from network name, in the following
+			// format: "/indexer/ingest/<network-name>"
+			TopicName:         "",
+			PurgeCacheOnStart: false,
 		},
 
 		Subsystems: MinerSubsystemConfig{

node/config/def_test.go

@@ -74,8 +74,8 @@ func TestDefaultMinerRoundtrip(t *testing.T) {
 	require.True(t, reflect.DeepEqual(c, c2))
 }
 
-func TestDefaultStorageMiner_SetsIndexIngestTopic(t *testing.T) {
+func TestDefaultStorageMiner_IsEmpty(t *testing.T) {
 	subject := DefaultStorageMiner()
 	require.True(t, subject.IndexProvider.Enable)
-	require.Equal(t, "/indexer/ingest/mainnet", subject.IndexProvider.TopicName)
+	require.Equal(t, "", subject.IndexProvider.TopicName)
 }

node/config/doc_gen.go

@@ -404,7 +404,9 @@ advertisements that include more multihashes than the configured EntriesChunkSiz
 			Type: "string",
 
 			Comment: `TopicName sets the topic name on which the changes to the advertised content are announced.
-Defaults to '/indexer/ingest/mainnet' if not specified.`,
+If not explicitly specified, the topic name is automatically inferred from the network name
+in following format: '/indexer/ingest/<network-name>'
+Defaults to empty, which implies the topic name is inferred from network name.`,
 		},
 		{
 			Name: "PurgeCacheOnStart",

node/config/types.go

@@ -186,7 +186,9 @@ type IndexProviderConfig struct {
 	EntriesChunkSize int
 
 	// TopicName sets the topic name on which the changes to the advertised content are announced.
-	// Defaults to '/indexer/ingest/mainnet' if not specified.
+	// If not explicitly specified, the topic name is automatically inferred from the network name
+	// in following format: '/indexer/ingest/<network-name>'
+	// Defaults to empty, which implies the topic name is inferred from network name.
 	TopicName string
 
 	// PurgeCacheOnStart sets whether to clear any cached entries chunks when the provider engine