[META] unified (ECS) target support in Tier-1 plugins

[kares]

This is a sub-task of #11635

Some of the plugins do not pollute the default name-space explicitly, however they accept arbitrary user data (e.g. json).
Which means in terms of ECS compatibility they should use a target name-space to avoid ECS name collisions.

To handle such plugins we should warn the user with a generic message in ecs_compatibility mode.

• a target check helper: Feat: introduce a target check helper logstash-plugins/logstash-mixin-ecs_compatibility_support#6
• logstash-filter-json Feat: check target is set in ECS mode logstash-plugins/logstash-filter-json#49
• logstash-filter-csv Refactor: unified ECS target + validate field reference logstash-plugins/logstash-filter-csv#86
• logstash-filter-kv Feat: check that target is set in ECS mode logstash-plugins/logstash-filter-kv#96
• logstash-codec-csv Feat: added target => namespace support + ECS compatibility logstash-plugins/logstash-codec-csv#7
• logstash-codec-json (also needs event_factory_support)
• logstash-codec-json_lines (also needs event_factory_support)
• logstash-codec-line
• logstash-codec-multiline
• logstash-codec-plain
• logstash-codec-avro add ECS support logstash-plugins/logstash-codec-avro#36

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NOTE: target support (wout ECS) has been added to several Tier 2 codecs ...