Test API coverage against the spec

ES has a well defined API spec here: https://github.com/elasticsearch/elasticsearch/tree/master/rest-api-spec/api

We should test our API coverage against specific versions. On that note, we should probably explicitly note which versions we support (where support means we strive for 100% coverage) and which we don't (coverage might be incomplete). We could even add methods in the client for determining whether the connected version of ES is officially supported.

Start by reviewing the Elasticsearch API specification at rest-api-spec/api and comparing it with the elastomer-client API surface. Determine how version-specific coverage should be measured and which Elasticsearch versions are officially supported. Done means the agreed coverage checks, support policy, and connected-version behavior are defined and implemented.

Vom Indexierungsmodell aus dem Issue-Text verfasst.