Add a proposed specification for an extension that returns the content
of a device image.

A couple changes to simplify and clarify this extension:

* Rename the functions to make it clear they get the "backend" content
  of the device image.  This will avoid confusion with a new member
  function `kernel_bundle::ext_oneapi_get_content` that we plan to add
  in a separate extension.

* Constrain the functions, so they are only available when the kernel
  bundle is `bundle_state::executable`.  This is the only case we need
  now, and it avoids the need to define what is returned in the other
  states.

Co-authored-by: John Pennycook <[email protected]>

Co-authored-by: John Pennycook <[email protected]>

Two changes that make it easier to use `device_image` by code that
does not also have the `kernel_bundle` object:

* Add an API to `device_image` that returns the backend.
* Change the lifetime of the content view to be the lifetime of the
  `device_image` object instead of the containing `kernel_bundle` object.