Add benchmarks for pallet-membership (#8596)

* Add benchmakrs for membership

* Update frame/membership/src/lib.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* Make it all work

* Add mock weights

* Update frame/membership/src/lib.rs

* Update frame/membership/src/lib.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* cargo run --release --features=runtime-benchmarks --manifest-path=bin/node/cli/Cargo.toml -- benchmark --chain=dev --steps=50 --repeat=20 --pallet=pallet_membership --extrinsic=* --execution=wasm --wasm-execution=compiled --heap-pages=4096 --output=./frame/membership/src/weights.rs --template=./.maintain/frame-weight-template.hbs

* cargo run --release --features=runtime-benchmarks --manifest-path=bin/node/cli/Cargo.toml -- benchmark --chain=dev --steps=50 --repeat=20 --pallet=pallet_membership --extrinsic=* --execution=wasm --wasm-execution=compiled --heap-pages=4096 --output=./frame/membership/src/weights.rs --template=./.maintain/frame-weight-template.hbs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>
Co-authored-by: Parity Benchmarking Bot <admin@parity.io>