Feature/improve codegeneration

[gmarz]

Before this change, endpoints that were removed from newer versions of the REST spec were subsequently removed from the low-level clients generated classes as well. We need to keep these endpoints in order to maintain backwards compatibility with older versions of elasticsearch.

All endpoints are now kept in the ApiEndpoints folder. This folder always contains the state in which the low-level client was generated from. Since files are never deleted from this folder, when an endpoint is removed from the REST spec, it's file will still remain.

When useCache == true, we skip downloading of the REST spec and generate directly from the existing contents of ApiEndpoints directory.

This change requires that the ApiEndpoints folder and it's contents are always checked in.

WIP: Need to fix build errors in NEST after updating to the 1.4 spec before this can be merged.

[Mpdreamz]

IndicesGet is new and has a new route parameter called feature that we need to add to PathInfo

https://github.com/elasticsearch/elasticsearch/blob/master/rest-api-spec/api/indices.get.json
elastic/elasticsearch@58f7b4a
elastic/elasticsearch#4069

get template

The body was dropped, need to figure out what happened here and how to support it in a bwc manner moving forward:

https://github.com/elasticsearch/elasticsearch/blob/v1.4.0/rest-api-spec/api/get_template.json
https://github.com/elasticsearch/elasticsearch/blob/v1.3.0/rest-api-spec/api/get_template.json
elastic/elasticsearch#7569

The descriptor form here: https://github.com/elasticsearch/elasticsearch-net/blob/633e663cdd11ef19d0a4c3cb730f954a2e097a7a/src/Nest/ElasticClient-SearchTemplate.cs#L110 sets .Name() while the dispatcher looks for .Id on the PathInfo whats going on there?

[gmarz]

Name acts as the search template id and get's assigned to Id in GetSearchTemplatePathInfo.Update(). I think the confusion stemmed from the fact that indices.get_template refers to the id as name, and we tried to model search templates the same way. I'm all +1 for changing name to id everywhere for search templates, which will probably require some sort of base IdPathDescriptor similar to NamePathDescriptor (unless I'm missing something), but this is a 2.0 thing.

I don't see why get_template would have a body; looks like a mistake in the 1.3 spec to me. I don't think we need to worry about BWC here...

[Mpdreamz]

LGTM 👍