Currently the `org.osbuild.files` input only supports the source origin.
Extend support to mapping files from pipelines, using the recently added
sub-tree reading capability of `ObjectStore.reat_at`. Restructure the
JSON schema to keep is as readable as possible.
The `org.osbuild.files` input provides individual files to a stage.
Change the `refs` key in the returned dict to `files` to better
reflect that fact. Also adapt the documentation to indicate that
the keys actually paths and not necessarily checksums. This prepares
for future extension of the `files` input to pipeline origins.
Create a `InputService` class with an abstract method called `map`,
meant to be implemented by all inputs. An `unmap` method may be
optionally overridden by inputs to cleanup resources.
Instantiate a `host.ServiceManager` in the `Stage.run` section and
pass the to the host side input code so it can be used to spawn the
input services.
Convert all existing inputs to the new service framework.
Instead of bind-mounting each individual input into the container,
create a temporary directory that is used by all inputs and bind-
mount this to the well known location ("/run/osbuild/inputs"). The
temporary directory is then passed to the input so that it can
make the requested resources available relative to that directory.
This is enforced by the common input handling code.
Additionally, pass the well known input path via a new "paths" key
to the arguments dictionary passed to the stage.
