Request Flow Customization (authentication interceptors)

[barend]

Fixes #66 .

This PR adds an interceptor API for hooking an extension into the HTTP traffic between Scruid and Druid. The API allows the simple case of intercepting the request before its transmission (as implemented by BasicAuthenticationExtension), as well as the complicated case of intercepting the response, reading an authentication challenge, reaching out to some other endpoint to obtain a credential, and finally re-issuing the request.

Implementation notes:

• The heavy lifting was done by @bjgbeelen and @anskarl; I mostly just added the Scaladocs.
• There's no built-in support for chaining interceptors, but the API allows for constructing a composite interceptor, which is left as an exercise to the reader.
• There is currently no Kerberos implementation. That's complex enough to warrant its own branch. A work-in-progress branch is in my fork, but this is nowhere close to being mergeable.
  • I removed the Kerberos Authenticator stub that Bas originally added to validate the async API. It's not ready, and shouldn't make it into production code.
• The testing harness uses an NGINX proxy to add basic auth. This is more convenient than updating the Druid config, which would require restarting Druid as part of the test run. We already had an NGINX instance, so it adds little or no weight to the testing harness.
• Unrelated changes:
  • Changed dependency scope of logback-classic to Provided, so that Scruid doesn't push it onto downstream projects as a transitive dependency.

[barend]

I renamed RequestFlowExtension to RequestInterceptor, which I think is a little easier on the eyes. This is slightly inaccurate, because it intercepts not just requests but also responses. However, I think people will figure that out quickly enough when the compiler tells them to implement the interceptResponse method.

[mmartina]

I renamed RequestFlowExtension to RequestInterceptor, ...

There are still some traces of classes named Xxx(flow)Extension. Is that on purpose?

[barend]

There are still some traces of classes named Xxx(flow)Extension. Is that on purpose?

@mmartina No, that was entirely by mistake. I have addressed this.

I have now (also) added the auth config to the documentation.