Create CustomIndexer module

[SupernaviX]

Create the scaffolding for a CustomIndexer module. See https://github.com/SundaeSwap-finance/acropolis-indexer-sample for what this module may look like.

Consumers will implement a ChainIndex trait with their own business logic. This trait will have methods which the CustomIndexer calls in response to chainsync events.

This module will not be used in omnibus, it's only intended to be embedded in external applications (i.e. the Sundae scooper). Unlike other Caryatid modules, the caller will construct an instance of this module and pass it to the Process at runtime. This gives them a simple way to provide their own index implementations.

For now, only support running a single "index" at a time.