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

documentation/en/default-lotus-miner-config.toml

@@ -291,11 +291,13 @@
   #EntriesChunkSize = 16384
 
   # 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.
   #
   # type: string
   # env var: LOTUS_INDEXPROVIDER_TOPICNAME
-  #TopicName = "/indexer/ingest/mainnet"
+  #TopicName = ""
 
   # PurgeCacheOnStart sets whether to clear any cached entries chunks when the provider engine
   # starts. By default, the cache is rehydrated from previously cached entries stored in