feat: Add a GO plugin for geo routing

[mateustd-ciandt]

This PR adds a GO 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.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

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

---

This comment is generated by snippet-bot.
If you find problems with this result, please file an issue at:
https://github.com/googleapis/repo-automation-bots/issues.
To update this comment, add snippet-bot:force-run label or use the checkbox below:

• Refresh this comment

[mpwarres]

Hmm, it seems that merging main into this PR broke something. @mateustd-ciandt could you take a look?

[mateustd-ciandt]

@mpwarres we should be good now!