add `ignore_missing` property to `foreach` processor

[djschny]

Elasticsearch version: v5.1.1

Description of the problem including expected versus actual behavior:

The foreach processor throws an error if the json element is not present in the document. Request to have it implement ignore_missing to resolve like what was done for other processors so that ignore_failure is not needed.

See #19995 for reference.

[elasticmachine]

Pinging @elastic/es-core-infra