Revise input checks in API functions

[lukpueh]

Description of issue or feature request:

While reviewing #364 I realized that we don't visibly sanitize all arguments at the top of all of our interface functions. This might be because we trust the internal functions, to which the interface functions in many cases pass the arguments on, to take care of malformed arguments.

I suggest to use our schema checker facility on all arguments of the public facing functions to fail early (also see secure-systems-lab/securesystemslib#183), especially in the light of #369, which officially defines our public API.

Current behavior:
Not all API functions sanitize all passed arguments at the user boundary.

Expected behavior:
Revise API / public-facing functions and add schema checkers for all arguments at the top, to fail early (or add comment where not feasible/desirable).
Revise internal non-public facing functions if some argument sanitization can be removed

See https://in-toto.readthedocs.io/en/latest/api.html and https://in-toto.readthedocs.io/en/latest/model.html for a list of all public facing functions and methods.