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
