Accept unknown properties #34
Description
To be forward compatible, we should accept unknown fields and array members. Doing otherwise would tie configuration files to a maximum version of this library, which would be an issue if the configuration files are written for a newer version.
We should be able to warn about such unknown properties and suggest users to update their library.
This will also align with OCI spec: https://github.com/opencontainers/runtime-spec/blob/main/config.md#extensibility
See opencontainers/runtime-spec#1241
We may want to still have a flag to switch to a stricter mode and deny unknown properties, which would be useful for tests and give guarantees for fully owned configurations. To ease composability, this should should also specify the scope of this enforcement: the current file, or all the (other) files part of the composition.