sightglass-next: implement sightglass-server

In order to report performance results based on PRs, we talked about implementing an HTTP server (e.g. in `crates/server`) that would:
- listen for incoming `POST` requests that contain JSON with the PR URL, commit SHA, etc. necessary for doing a "master vs PR" comparison
- to avoid DoS, verify that the request is an authorized one (not exactly sure how to do this but the GitHub action will need some form of token)
- kick off some benchmark running, like `sightglass-cli benchmark ...` but we could call the same APIs from inside the web service
- upon success, push a Markdown table of the significant performance differences as a comment to the GitHub PR (implies that `sightglass-server` has a GitHub token, like one from `bytecodealliance-highfive`)
- upon failure, push the error message as a comment to the GitHub PR (implies that we maintain error logs somewhere)

There is a lot more to be done here but that should be a workable start.