Adds two new methods to `RestClient` that take a `Request` object. These
methods will allows us to add more per-request customizable options
without creating more and more and more overloads of the `performRequest`
and `performRequestAsync` methods. These new methods look like:
```
Response performRequest(Request request)
```
and
```
void performRequestAsync(Request request, ResponseListener responseListener)
```
This change doesn't add any actual features but enables adding things like
per request timeouts and per request node selectors. This change *does*
rework the `HighLevelRestClient` and its tests to use these new `Request`
objects and it does update the docs.
0be443c5bb