[Meta] Standardize DataStream handling / config parsing

[andrewvc]

In #24223 I introduced a new add_data_stream_index processor that provides a very clean way for handling data stream configs and data stream index formatting. Today that's only used for Heartbeat, but as @urso pointed out a similar strategy would make sense for the other beats as well, probably leveraging some of the same utilities.

The basic idea is to centralize two bits of logic:

1. DataStream configs (which exist at both the input and the stream level)
2. The index name generation

The processor above handles the index name generation (and allows individual events to override the dataset via metadata). The first bit of logic, merging the dual data_stream declarations, is handled in a bespoke way for heartbeat, but could probably be centralized with a helper function after applying it to one other beat to find the commonalities.

This issue exists to track this unification effort.

The below list tracks implementation across the following beats:

• Filebeat
• Packetbeat
• Metricbeat
• APM Server

[botelastic]

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