Add benchmarking for parachain runtime initializer pallet (#3913)

* Add benchmarking for parachain runtime initializer pallet

* Fix default impl for initializer pallet WeightInfo

* cargo run --quiet --release --features=runtime-benchmarks -- benchmark --chain=westend-dev --steps=50 --repeat=20 --pallet=runtime_parachains::initializer --extrinsic=* --execution=wasm --wasm-execution=compiled --heap-pages=4096 --header=./file_header.txt --output=./runtime/westend/src/weights/runtime_parachains_initializer.rs

* cargo run --quiet --release --features=runtime-benchmarks -- benchmark --chain=kusama-dev --steps=50 --repeat=20 --pallet=runtime_parachains::initializer --extrinsic=* --execution=wasm --wasm-execution=compiled --heap-pages=4096 --header=./file_header.txt --output=./runtime/kusama/src/weights/runtime_parachains_initializer.rs

* Use real weights and use max_block as default weight

* Add variable for digest vec length for initializer benchmark

* Fix compilation errors

* Add WeightInfo to parachains_initializer config in polkadot runtime

* cargo run --quiet --release --features=runtime-benchmarks -- benchmark --chain=westend-dev --steps=50 --repeat=20 --pallet=runtime_parachains::initializer --extrinsic=* --execution=wasm --wasm-execution=compiled --heap-pages=4096 --header=./file_header.txt --output=./runtime/westend/src/weights/runtime_parachains_initializer.rs

* cargo run --quiet --release --features=runtime-benchmarks -- benchmark --chain=kusama-dev --steps=50 --repeat=20 --pallet=runtime_parachains::initializer --extrinsic=* --execution=wasm --wasm-execution=compiled --heap-pages=4096 --header=./file_header.txt --output=./runtime/kusama/src/weights/runtime_parachains_initializer.rs

* Appease spellchecker

* Use kusama weights in polkadot runtime for parachain initializer pallet

Co-authored-by: Parity Bot <admin@parity.io>
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