Add contract instance to ledger entry fetch

[elizabethengelman]

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What

this pr is based off of #2012

closes https://github.com/stellar/stellar-cli/issues/2273

Updates the ledger entry fetch command to allow for returning the contract instance ledger entry by adding an --instance option which will include the contract instance ledger entry to the list of keys being fetched.

cargo run ledger entry fetch contract-data --contract hello-world --network local --instance

Why

This way we are able to more easily fetch a contract wasm hash based on it's contract id/alias.

Known limitations

[TODO or N/A]