Add cache-busting and server headers with a wrapper.

[NoraCodes]

In order to support custom headers for various response types, this commit adds a wrapper type, ResponseWrapper, which can service all types of response in bin.

For paste objects, the preferred Last-Modified is used, so that caches can compare their exact timings with the HEAD response when revalidating.

For static objects, an ETag is used instead, based on the Cargo version and git hash of the codebase at compilation time; a build.rs is used for this.

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• [x] Cargo Format

  • Run cargo fmt on the project.

• [x] Clippy lints

  • Run cargo clippy -- -Dwarnings on the project to check for suggestions
  • Run cargo clippy --fix to let clippy apply the suggestions itself, if any.

[wantguns]

Thanks for the PR ! It surely sets a strong foundation for any upcoming changes. I am gonna merge this as is, those comments were more like notes.