Refactored type definitions (#1119)

* Updated types generation script

* Refactored api method definitions

* Updated test
- Removed old test code
- Added tsd dev dependency
- Rewritten test with tsd

* Removed unused dependencies

* Fixed definition

* Updated test

* Updated docs

* Improved events type definitions

* Updated test

* Minor fixes in the type definitons

* More type test

* Improved Transport type definitions

* Updated test

* Addressed comments

* Code generation

* Use RequestBody, Response and Context everywhere, also default Context to unknown

* Updated test

* body -> hasBody

* Fixed conflicts

* Updated code generation

* Improved request body type definition

* Updated code generation

* Use BodyType for both request and reponses generics
- Use extends for defining the RequestBody generic to force the user
  following the same shape.
- BodyType and NDBodyType now accepts a generics to allow injecting
  more specific types in the future

* API generation

* Updated test

* Updated docs

* Use BodyType also in ReponseError

* Removed useless client generics

* Renamed generics and types
- prefixed all generics with a T
- BodyType => RequestBody
- NDBodyType => RequestNDBody
- Added ResponseBody

* Updated test

* Updated docs

* Test ResponseBody as well

* Simplify overloads

* API generation

* Updated test

* Updated error types

test/types/api-response.test-d.ts

@@ -0,0 +1,35 @@
+// Licensed to Elasticsearch B.V under one or more agreements.
+// Elasticsearch B.V licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information
+
+import { expectType } from 'tsd'
+import { ResponseBody } from '../../lib/Transport'
+import { Client } from '../../'
+
+const client = new Client({
+  node: 'http://localhost:9200'
+})
+
+// No generics
+{
+  const response = await client.cat.count({ index: 'test' })
+
+  expectType<ResponseBody>(response.body)
+  expectType<unknown>(response.meta.context)
+}
+
+// Define only the request body
+{
+  const response = await client.cat.count<string>({ index: 'test' })
+
+  expectType<string>(response.body)
+  expectType<unknown>(response.meta.context)
+}
+
+// Define request body and  the context
+{
+  const response = await client.cat.count<string, string>({ index: 'test' })
+
+  expectType<string>(response.body)
+  expectType<string>(response.meta.context)
+}