Implement ECS-Compatibility Mode

[yaauie]

This is a stub issue, and needs to be fleshed out with details specific to
this plugin.

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As a part of the effort to make plugins able to run in an ECS-Compatible manner
by default in an upcoming release of Logstash, this plugin needs to either
implement an ECS-Compatibility mode or certify that it does not implicitly use
fields that conflict with ECS.

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Introduce a target option that specifies the name of the sub-field where nest the row columns loaded from DB. This option should be populated if ECS support is not disabled.

[andsel]

@yaauie PR #69 with fixes for JDBC input and PR #71 with fixes for JDBC filter static were published with plugin version 5.1.0.
For the JDBC filter streaming the target is mandatory, so we can close this issue.

[andsel]

I close this since with version 5.1.0 of logstash-integration-jdbc ship it