Refactor alliance benchmarks to v2 (paritytech#1868)

- This PR refactors `alliance/src/benchmarkings.rs` to use benchmarking
v2. These changes are needed to improve the readability and
maintainability of the benchmarking code.

- No known issue to backlink.

## Local Testing 
1. `cargo build --features runtime-benchmarks` 
2. `cargo run --locked --release -p node-cli --bin substrate-node
--features runtime-benchmarks -- benchmark pallet --execution wasm
--wasm-execution compiled --chain dev --pallet "*" --extrinsic "*"
--steps 2 --repeat 1`