[rust-server] Implement stack for Context

Created by: bjgill

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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 https://github.com/Metaswitch/swagger-rs/pull/36 for the associated swagger-rs changes.

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