feat: integrate stellar-strkey for key validation (closes #7)

[Idrhas]

Closes #7

What this adds

Cargo.toml

• Added stellar-strkey = "0.0.8" for StrKey encoding/decoding
• Added ed25519-dalek = "2" for public key derivation from secret seeds

src/utils/mod.rs (new)

• Exposes pub mod keypair

src/utils/keypair.rs (new)

• is_valid_public_key(key: &str) -> bool — validates G-prefixed Stellar public keys
• is_valid_secret_key(key: &str) -> bool — validates S-prefixed Stellar secret keys
• public_key_from_secret(secret: &str) -> Result<String, KeyError> — derives public key from secret
• KeyError enum with InvalidSecretKey and InvalidPublicKey variants
• 18 unit tests covering valid keys, invalid inputs, wrong prefix, truncated strings, cross-type mismatches, and correct derivation

src/main.rs

• Added pub mod utils