Benchmark Im Online Pallet (#5318)

* Initial benchmarking setup

* Add keystore

* validate unsigned

* Update frame/im-online/src/benchmarking.rs

Co-Authored-By: Marcio Diaz <marcio.diaz@gmail.com>

* Fix verify_unsigned benchmark

* add variable for teting the external addresss length

* Update frame/im-online/src/benchmarking.rs

Co-Authored-By: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Update utils/frame/benchmarking-cli/src/lib.rs

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

Co-authored-by: Marcio Diaz <marcio.diaz@gmail.com>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-authored-by: Benjamin Kampmann <ben@gnunicorn.org>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/Cargo.lock

@@ -1449,6 +1449,7 @@ dependencies = [
  "sc-executor",
  "sc-service",
  "sp-core",
+ "sp-externalities",
  "sp-runtime",
  "sp-state-machine",
  "structopt",
@@ -4225,6 +4226,7 @@ dependencies = [
 name = "pallet-im-online"
 version = "2.0.0-alpha.4"
 dependencies = [
+ "frame-benchmarking",
  "frame-support",
  "frame-system",
  "pallet-authorship",