Deploy gas token as part of sandbox bootstrap

[alexghr]

Deploy the gas token when the sandbox is started. Related to #4801 but for the sandbox, instead of end-to-end test.

Deploying the contract requires two steps:

• first registering the Gas token contract class registerContractClass
• then deploying an instance of the contract deployInstance

Ideally these two steps would be batched together into a single tx. An appropriate wallet would have to be used (signerless wallet only supports a single function call per tx instead of a batch).