Indexed Scripts : Support for indexed scripts.

Scripts can now be indexed like templates.
Anything that accepted a "script" argument now also supports an "id" or "file" argument.
For backwards compatiblity files referenced from the "script" argument are supported.
Indexed scripts now have CRUD endpoints under /_search/scripts/{lang}/{id} similar to templates.
Added a test for on disk scripts that was missing.

See #5484 #5921