refactor(rust): refactored identity show command to use rpc abstraction

[divyank-aggarwal]

Current Behavior

The current implementation in identity show command is calling std::process::exit, which is something we want to stop using to handle errors properly and give the user a better-formatted output when a command fails.

Proposed Changes

Relating to issue #3562

1. Changed the implementation to use the node_rpc method as being used in subscription command
2. Added implementations of ockam_command::util::output for ShortIdentityResponse and LongIdentityResponse to maintain current code quality and style
3. Marked 2 functions that were being used before as allow(unused) to prevent linting errors.

Checks

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[adrianbenavides]

Looking great @divyank-rs! A couple of things to address:

1. You can delete the unused methods
2. Can you add the bats test you mentioned here?

[adrianbenavides]

Awesome, thank you so much @divyank-rs for your contribution! 🎉

[mrinalwadhwa]

@divyank-rs thank you for an awesome first contribution 🥳