[#323] Add `parseSecretKey` for secp256k1 and P256 crypto

Description

Problem: parseSecretKey is missing for secp256k1 and P256 crypto.

Solution: Add parseSecretKey for secp256k1 and P256 crypto, modifying SecretKey to use PrivateKey instead of KeyPair, and add the tests.

Related issue(s)

Resolves #323 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.