Remove dependency on jwcrypto and cryptography

[angela-tran]

This task is to make the code changes necessary for removing jwcrypto from our requirements.txt file. (cryptography is a transitive dependency of jwcrypto, so we can remove that as well.) Some of our Django models currently return jwk.JWK instances in some of their methods.

Acceptance Criteria

• The requirements.txt file does not contain jwcrypto or any of its dependencies
• The Benefits app still works as expected
• Unit tests still pass

Additional context

#141 mentions that a goal is to consolidate dependency management, which means benefits should not be directly depending on packages that are only needed for the Eligibility API. #596 pulls out most of the usage of jwcrypto, except for the ones left in core/models.py and tests/pytests/eligibility/test_views.py.

[thekaveman]

Commented on cal-itp/eligibility-api#31, which I think does more than necessary to solve this.

My thinking for solving this issue was actually more to simplify the benefits side - why do the models even need to return that JWK type? They are just passing directly into this API - it should fully internalize the logic and deal with the raw underlying type used in the model (e.g. the text or the bytes).

We don't really need a special class here.