feat: create a Rust sample for overwrite header plugin.

[walves-cit]

This plugin is a overwriting request and response header showcase.

Technically, this is performed by replacing some existing header according to the desired key.

This examples contains only a Rust version.

[snippet-bot]

Here is the summary of changes.

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• plugins/samples/overwrite_header/plugin.rs:15, tag serviceextensions_plugin_overwrite_header

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