chore: remove derived `Debug` and `Serialize` from private key types

[mrain]

closes: #694

This PR:

In jf-signature crate

• Manually implemented Debug for private signing keys to not print the actual value.
• Remove derived (de)serialization. Implemented to/from_bytes() and the conversions to/from TaggedBase64.

This PR does not:

• Private key type in elgamal crate still has derivations. Because of this ephemeral key in the ciphertext: https://github.com/EspressoSystems/jellyfish/blob/5f06aa175e8d49100203b027c08aa8d174e3f0d4/elgamal/src/lib.rs#L147

• Private key type in aead crate still has derivations. Because of this ephemeral key in the ciphertext: https://github.com/EspressoSystems/jellyfish/blob/5f06aa175e8d49100203b027c08aa8d174e3f0d4/aead/src/lib.rs#L279

• jf-vrf crate should be updated after to adopt these changes in jf-signature.

Key places to review:

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• [ ] Wrote unit tests
• [x] Updated relevant documentation in the code
• [x] Added relevant changelog entries to the CHANGELOG.md of touched crates.
• [ ] Re-reviewed Files changed in the GitHub PR explorer