out_syslog: handle multivalued structured data

[bplessis]

When generating a syslog SD-ELEMENT it is permitted to repeat a SD-PARAM multiple times: https://datatracker.ietf.org/doc/html/rfc5424#section-6.3.3

This is used by loggly to pass tags alongside syslog messages for example https://documentation.solarwinds.com/en/success_center/loggly/content/admin/tags.htm

This proposal convert array-typed object into multiple SD-PARAM elements to be able to satisfy this need.

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