Refactor Identity to benchmark v2 (paritytech#1838)

This PR refactors `identity/benchmarkings.rs` to use benchmarking v2.
These changes are needed to improve the readability and maintainability
of the benchmarking code. Changes were implemented using
[this](paritytech@9ec8009)
commit as a guide. The logic of the benchmarks remains the same.

No known issue to backlink.

## Local Testing
To test the new benchmarks:
1. `cargo build --features runtime-benchmarks`
2. `./target/debug/polkadot benchmark pallet --steps=5 --repeat=2
--pallet=pallet_identity --extrinsic='*'`

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Co-authored-by: Richard Melkonian <[email protected]>
Co-authored-by: joe petrowski <[email protected]>
Co-authored-by: command-bot <>
Co-authored-by: Francisco Aguirre <[email protected]>
Co-authored-by: Oliver Tale-Yazdi <[email protected]>