Create explicit crypto_null stub files for crypto platform functionality.

[taprinz]

Part of a long series of planned changes to address #10. As spdm_ring doesn't build on riscv64, we cannot use it as a crypto implementation backend. In general, an integrator of the library onto an embedded platform (like riscv64) will want to implement their own crypto hooks, for example with HW accelerators.

Because of this, we want to make it obvious for an integrator as to what functions they would need to provide in order to create their own platform implementation. This change does not modify functionality so much as it does serve as a minor refactor of where the NULL implementations of crypto platform APIs are located. This is done to be more intuitive to an integrator as to how they could add another crypto library, and what functions they would need to implement.

In the long run, it would be desirable to create similar null or stub implementations of all platform functionalities which rely on external crates which may not be usable on an embedded platform.

Note that I did not add crypto_null as a feature, as it seems Rust features are meant to only be additive, and not require mutual exclusivity (as crypto backends would require). This would be an issue in the long term if we are to formally support another crypto implementation upstream, outside of spdm-ring.

[taprinz]

Do we want to update/remove the copyright?