Allow for fetching contract instance with `ledger entry fetch` more easily

[elizabethengelman]

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I started to spike this out in #2269 - with the addition of stellar ledger entry fetch we should be able to fetch a contract's instance ledger entry. In order to do this, we need to create a ScVal::LedgerKeyContractInstance as the key. The current shape of the ledger entry fetch command looks like this

stellar ledger entry fetch contract-data --contract hello-world --network local --key <KEY>

One idea, is that this command could accept "instance" as the key, and it would then build a ScVal::LedgerKeyContractInstance and use that to fetch the ledger entry.