[Task (Infrastructure)]: Consolidate indexer processors

[kodemartin]

Description

Following #1845 we have two modules in iota-indexer with overlapping scopes.

This creates confusion on what type/function/trait etc. should be where. Handlers process data, as well as processors in a way.

We should care of making this less ambiguous. Therefore I think it would be plausible to consolidate everything into a single module (handlers or processors), remove any suffixes/prefixes handlers or processors and classify between metrics in a nested module (what is now in the processors module except for the ObjectSnapshotProcessor) and the rest.