Maintenance: Use `offsetof()` instead of manual offset calculation

### Description

It's fragile to manually calculate offsets within structs like so, which the current code does in `psa_parse_key_data_from_storage()`.

```GET_UINT32_LE(policy->alg2, storage_format->policy, 2 * sizeof( uint32_t ));```

We should instead look into using `offsetof()`. This will make the library more portable and easier to maintain. it's not likely to be "more correct", since we have tests that would catch these types of padding issues in this instance.

### Issue request type
[ ] Question
[X] Enhancement
[ ] Bug

Start at psa_parse_key_data_from_storage() and inspect the manual offset use shown in the issue, including the GET_UINT32_LE call. Review the existing tests for struct padding and storage parsing; done means the relevant manual struct offsets use offsetof() and those tests still pass.

Written by the indexing model from the issue text.