Finalize separation of EC keys used for DSA and ECDH purposes

Naming and placement of this struct feels a bit off here - i.e., the key used for ECDH probably shouldn't be under the `dsa` module.

We should probably update the module structure a bit:

- This struct could go under `ecdh::k256` module.
- Once there, we could also rename it into something like `PersistentSecretKey` (to be consistent with `EphemeralSecretKey`).

Separately, we may want to do the same with public keys so that we don't conflate public keys used for key exchange vs. the ones used for signature verification.

_Originally posted by @bobbinth in [#965](https://github.com/0xMiden/crypto/pull/965/changes#r3105818252)_

The above should also be applied to the Curve25519 keys.