MultiGetElasticsearch processor should support regex format, not only '*'

[mariemat]

Expected behavior and actual behavior.

Currently the MultiGetElasticsearch processor allows the metacharacter '*'. While this is a great way to enhance the selection of fields, this might not always be sufficient. It would be great to support the full Java regexp syntax to be extremely precise in the way to select fields.

Steps to reproduce the problem.

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Specifications like the version of the project, operating system, or hardware.

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