feat: add Rust implementation and dependencies for cdn_token_generator plugin

[mateustd-ciandt]

This PR adds the Rust version of the CDN Token Generator, a Proxy-WASM sample plugin designed to authenticate media requests for Google Cloud Media CDN.

It works by intercepting HTTP response bodies, such as video streaming manifests (HLS or DASH), and dynamically appending HMAC-SHA256 signed tokens to any embedded URLs it finds before sending the response back to the client.

We're using protobuf configuration to securely load the secret key and token expiration settings only once during initialization. Additionally, this PR includes the necessary Rust dependencies (such as hmac, sha2, and base64) and their respective Bazel build configurations.

[snippet-bot]

Here is the summary of changes.

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• plugins/samples/cdn_token_generator/plugin.rs:15, tag serviceextensions_plugin_cdn_token_generator_rust

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