Add address_field config option to handle nested fields

[estolfo]

Add a new config option, address_field that is a field reference for addresses that should be checked.
This handles the case when the event has nested addresses, such as the following:

"host": {
      "ip": [
        "fe80::xxxx:xxxx:xxxx:xxx",
        "10.x.x.x",
        "fe80::xxxx:xxxx:xxxx:xxx",
        "10.x.x.x"
      ],
      "name": "HOSTNAME"
    },

With the current config option address, it wasn't possible to define that all addresses in the ip array should be checked.

Resolves #19

[Copilot]

Pull Request Overview

This PR adds a new address_field configuration option to the CIDR filter plugin, allowing users to specify a field reference for IP addresses that should be checked against network blocks. This enhancement enables handling of nested IP address arrays in event data structures.

Key changes:

• Introduces address_field config option as an alternative to the existing address option
• Adds mutual exclusivity validation between address and address_field configurations
• Refactors IP address extraction logic to support both field references and template strings

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
lib/logstash/filters/cidr.rb	Core implementation of address_field functionality and refactored address handling
spec/filters/cidr_spec.rb	Comprehensive test coverage for the new address_field option
logstash-filter-cidr.gemspec	Added dependency for field reference validation
docs/index.asciidoc	Documentation for the new address_field configuration option

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[yaauie]

This looks to be on the right track.

I've suggested adding some specs for the behaviour when encountering events that aren't shaped correctly (though I have not run them locally).

[yaauie]

The changes and specs look good.

We will need to bump the minor version and add changelog entry.

[yaauie]

👍🏼 LGTM