Downgrade bundle's abstraction level

- Symfony version: 4
- Algolia Search Bundle version: 3.1.2
- Algolia Client Version: 1.25.1
- Language Version: ru

### Description
Bundle has too high abstraction level by deciding which index to use with search or index ops. Any entity may be placed in different indexes (e.g. different order). Same problem taking place in discriminated doctrine entities, that could be indexed separately or all together.

My suggestion is to downgrade abstraction level for IndexManager and let developers write their own SearchIndexerSubscribers, to pass required indexes and actions.

Start by reading the IndexManager and SearchIndexerSubscribers mentioned in the issue, then trace how the bundle currently selects indexes for search and indexing operations. Compare this with discriminated Doctrine entities and identify the extension points needed for developers to provide indexes and actions explicitly. Done means the abstraction is reduced without leaving existing indexing flows undefined.

Written by the indexing model from the issue text.