Add Stellar Vault Contract

[jsmaxi]

Feature Request:
Include Stellar Vault contract example in the UI Wizard, alongside the existing Fungible, NonFungible and Stablecoin contracts.

• Wizard UI: https://wizard.openzeppelin.com/stellar
• Example Implementation: https://github.com/OpenZeppelin/stellar-contracts/blob/main/examples/fungible-vault/src/contract.rs