The truthiness of the `Schema` object itself now contains the
schema validation as well, i.e. schema is only valid if schema
information is present and said information passes validation.
The _validator member of `Schema` is used as an indicator whether
the provided schema is valid. The `check` method will, in case
that _validator is not set attempt to validate the schema data,
if present and set the _validator member if schema data is set and
validation has passed. On failure, i.e. missing schema information
or invalid schema data, the ValidationResult will contain the
respective error.
This new module contains utilities that help to introspect parts
that constitute the inner parts of osbuild, i.e. its stages
and assembler (which is also considered a type of stage in
this context). It contains the `StageInfo` class that can that
contains meta-information about the individual stage, such as
a short information (`info`), a longer description (`desc`) and
its JSON schema. A new Schema class represents schema data and
has a `validation` method that can be used to validate that json
data conforms to said schema.
A `Index` class can be used to obtain `StageInfo` and `Schema`
for entities identified via `klass` and `name`.
A top level `validate` method is introduced that can validate
manifest data.
Internally it uses the `jsonschema` package so add that as a
requirement and Install this dependency in the CI.
