feat: create a Rust sample plugin for creating a new cookie on HTTP response.

[pweiber]

Checks for an existing my_cookie in incoming HTTP requests:

If the cookie exists, it logs the current session ID.
If absent, it generates a new random session ID, sets it in the Set-Cookie header, and logs the creation of the new session.

This example contains only a Rust version.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• plugins/samples/set_cookie/plugin.rs:15, tag serviceextensions_plugin_set_cookie

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