Arbitrary filters for fetching events from the indexers

[dzhelezov]

Current state of the art

The processor analyzes all the events available for the mappings and fetches from the indexer all the events that match the name. Namely, if there are mappers named system_ExtrinsicSuccess and treasury_Deposit the processor queries the indexer for all events with the names system.ExtrinsicSuccess and treasury.Deposit

Proposal

Use a more holistic approach for fetching the events by defining a more flexible filter for fetching the events. For starters, we may introduce filtering based on the name patterns (already supported by the indexer-api-gateway)