Add internal API for the nfd-worker custom feature source rule
configuration. This API has already diverged from the NFD
NodeFeatureRule API in that annotations, extended resources or taints
are not supported. This patch basically copies the Rule type (and it's
sub-types) from the nfdv1alpha1 package. It also adds conversion
functions from the internal rule API to the "external" nfdv1alpha1 API.
This is done to use the same rule matching functionality (from the
nfdv1alpha1 package).
One notable remark is that the feature source rule config supports some
custom formatting (short forms, multi-type fields) that relies on
special json/yaml unmarshalling functions that are better to nuke from
the nfdv1alpha1 package (in another patch). These (legacy) syntax
specialities are most probably used by nobody but let's keep them as
they're already there. Unit tests to cover the custom json
unmarshalling are now added.
