Add address book contract example

[alexgduarte]

Summary

Adds an address-book CosmWasm example contract. The contract demonstrates owner-managed state with named contacts, validated addresses, optional notes, ownership transfer, single-contact queries, and paginated list queries.

This is an original simple-tier contract example submitted for #11.

What changed

• Added address-book with instantiate, execute, query, state, message, and error modules.
• Included owner-only add/remove/transfer ownership behavior.
• Added unit coverage for default owner setup, contact add/query, unauthorized writes, pagination, and removal.
• Linked the new example from the root README.

Verification

• cargo fmt
• cargo test
• git diff --check

Bounty

Submitted for the simple contract tier in #11. GitHub contact: @alexgduarte. Payment details can be coordinated privately if accepted.