feat: implement kyc verifier contract

[Ndanusa]

Description

This PR introduces a dedicated contract for storing and verifying KYC status signatures on-chain, ensuring users have been verified by a trusted provider before interacting with sensitive protocol features.

Requirements Covered

• Elliptic curve signature verification (Ed25519)
• Secure storage for user KYC expiry timestamps
• Admin management for the trusted verifier public key

Technical Implementation

• Signature Verification: Uses env.crypto().ed25519_verify to validate a message composed of the user's Address and an expires_at timestamp.
• XDR Data Format: Converts addresses to XDR format consistently with the SDK version to ensure deterministic signature matching.
• Graceful Expiry: Includes an is_kyc_valid helper that checks both the existence of a KYC record and the current ledger time against the record's expiry date.

Testing

• Verified signature verification process and rejection of invalid signatures.
• Verified expiry logic correctly returns false for outdated proofs.

Closes #26

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[Ndanusa]

@Ceejaytech25 , pls review

[Ceejaytech25]

@Ndanusa , lgtm!