* Update the REST API specification
This patch updates the REST API spefication in JSON files to better encode deprecated entities,
to improve specification of URL paths, and to open up the schema for future extensions.
Notably, it changes the `paths` from a list of strings to a list of objects, where each
particular object encodes all the information for this particular path: the `parts` and the `methods`.
Among the benefits of this approach is eg. encoding the difference between using the `PUT` and `POST`
methods in the Index API, to either use a specific document ID, or let Elasticsearch generate one.
Also `documentation` becomes an object that supports an `url` and also a `description` which is a
new field.
* Adapt YAML runner to new REST API specification format
The logic for choosing the path to use when running tests has been
simplified, as a consequence of the path parts being listed under each
path in the spec. The special case for create and index has been removed.
Also the parsing code has been hardened so that errors are thrown earlier
when the structure of the spec differs from what expected, and their
error messages should be more helpful.
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