Add a helper function `JsonWebKey::set_key_ops` for setting JsonWebKey
key_ops attribute to a given list of key usages. This task is very
common in export key operation of different cryptographic algorithms.
Adding this helper function helps simplify our code.
Testing: Refactoring. Existing tests suffice.
Signed-off-by: Kingsley Yung <kingsley@kkoyung.dev>