Make RestSearchAction#parseSearchXXX(RestRequest) public
When building a plugin with a new search endpoint, you need to parse the request as a searchRequest.

Methods exist in RestSearchAction class but are private.

We will modify them to be public static. This applies to:

* `RestSearchAction#parseSearchRequest(RestRequest)`
* `RestSearchAction#parseSearchSource(RestRequest)`

 Closes #3499.