Add hex encoding and decoding (to_hex / from_hex)

[davidgaspardev]

Description

Implement functions to encode raw bytes to hexadecimal string representation and decode them back.

// {0x48, 0x69} → "4869"
byte_t to_hex(cbytes_t src, unsigned int src_len, bytes_t dst, unsigned long sizeof_dst);

// "4869" → {0x48, 0x69}
byte_t from_hex(cbytes_t hex, bytes_t dst, unsigned long sizeof_dst);

Use cases

• Debug output of binary buffers
• Network protocol inspection
• Cryptographic hash display (MD5, SHA digests)

References

• Encoding: Hexadecimal (Base-16)
• Standard: Each byte is represented as two hex digits (nibbles)
• Wikipedia: Hexadecimal
• Related: xxd, hexdump Unix tools