Add dynamic automatic test coverage of Config classes.

[myronmarston]

The shared example group in gem_spec.rb automatically applies to each ElasticGraph gem. The changes here:

• Verify that all config classes are defined using ElasticGraph::Config
• Verify that each config class has a valid JSON schema.
• Verify that each config class provides a default value for any properties which are not required.
• Verify that all default and examples values are valid.
• Verify that every property has a description and examples (which I plan to use in our docs later).
• Verify that each config class has a unique path.

For now, none of our config classes have been converted to use the new ElasticGraph::Config base class, so I've marked those specs as pending. We'll convert each config class in a later PR.

[CLAassistant]

All committers have signed the CLA.

[zachbutler-squareup]

Does this need its own tests? The logic is complex and feature-packed enough that it almost needs its own tests to validate that it validates correctly.

[myronmarston]

Great question. This runs on each CI build and we can see it passing there for confidence that it's not throwing exceptions or failing, but of course that doesn't demonstrate the expected failures when one of the config classes doesn't satisfy the requirements. If this was going to be released as part of ElasticGraph so that it applies to user code, I think that kind of test coverage would be warranted, but for a case like this (where it just runs as part of the test suite here, on the specific class definitions in this code base) I think there's unlikely to be a positive ROI on that investment.

I did get expected failures from each of the expectations in this test logic while working up each PR that migrates a gem to use the new ElasticGraph::Config class, so that gives me a good level of confidence that it works as expected.