[rust-server] Implement stack for Context

[bjgill]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.1.x, 4.0.x. Default: master.
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Description of the PR

This PR adds a SwaggerService trait that makes it much easier to compose multiple middlewares. This single common trait means that middlewares need less boilerplate. This change has been prompted by our experiences with an internal authentication middleware.

This PR also renames auth::Server to context::AddContext, to make the purpose clearer. Given that this is a breaking change (albeit a pretty trivial rename, as the example integration shows), I don't know if I should be filing this against master or 3.2.x...

See Metaswitch/swagger-rs#36 for the associated swagger-rs changes.

Thanks to @jngorman, the original contributor of this commit.

[jmini]

Changed base branch: 3.2.0-SNAPSHOT is now on master branch

[wing328]

@bjgill when you've time, I wonder if you can resolve the merge conflicts and then I'll merge it into master (3.2.0)

[bjgill]

Thanks. Now rebased and awaiting CI.

[bjgill]

CI all passed. Happy to get merged when you're ready.