feat: Add a cpp plugin for geo routing

[mateustd-ciandt]

This PR adds a C++ plugin that reads the client's country code from load balancer properties (request.client_region) and sets it as an x-country-code request header.
This header enables URL Map routing rules to direct traffic to geographically appropriate backend services.

The plugin includes security measures to prevent header spoofing and is optimized for performance (~55-58μs latency per request following the quotas and limits for Service Extensions.).

[snippet-bot]

Here is the summary of changes.

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• plugins/samples/geo_directional_origin/plugin.cc:15, tag serviceextensions_plugin_geo_routing

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[mpwarres]

Merge from main broke this, @mateustd-ciandt can you take a look?

[mateustd-ciandt]

@mpwarres this one should be good too, let me know if we run into any build files issues again!