Add an example contract demonstrating staking capability

[lumtis]

Functionality in zeta-chain/node#2754 gives the ability stake/claim ZETA through smart contracts.

This is a functionality that can brings lot of potential applications ideas (staking directly by making calls from Bitcoin, etc...) and it would be interesting to bring a example contract that can demonstrates these abilities.

To go further, maybe we could think about a single example contract incorporating the functionalities of the Universal staking idea: zeta-chain/node#2793

It brings two other functionalities:

• Use a distribution smart contract allow to send ZRC20 to be distributed to stakers
• Stakers earns rewards in ZRC20s from the cross chain fees

Staking is to be completed soon, while these two features above will be completed in a further release.

[fadeev]

Blocked until staking precompile is enabled on testnet.