Add support for loading private key from hex string

[saurabh-router]

• Resolves: #

Summary

This PR introduces a new function NewPrivateKeyFromHex in the private_key.go file. This function allows creating a PrivateKey instance from a hexadecimal string representation of the key.

Key changes:

1. Added a new function NewPrivateKeyFromHex that takes a hex string and a key algorithm as input.
2. Implemented support for both ED25519 and SECP256K1 algorithms.
3. Improved error handling with more descriptive error messages.
4. Ensured consistency with existing code style and patterns in the file.

Details:

• The function uses a switch statement to handle different key algorithms.
• It calls newly implemented NewPrivateKeyFromHex functions in the ed25519 and secp256k1 packages (added in the same commit).
• Proper error handling is implemented for unsupported algorithms and key creation failures.
• The public key is derived from the private key and combined with the algorithm byte.
• The function returns a PrivateKey struct with the algorithm, public key, and private key information.

This addition enhances the flexibility of the keypair package by allowing users to create private keys from hexadecimal strings, which is a common format for storing and transmitting cryptographic keys.

Checklist

• Code is properly formatted
• All commits are signed
• Tests included
• Documentation (manuals or wiki) has been updated or is not required

[saurabh-router]

@ihor could you please review this PR?