Benchmark Template for `election_provider_multi_block_*` pallets

[kianenigma]

as pointed out by @bkontur here, they need a custom template that injects a single line into the template.

Let's fix them manually for now to get things done for KAH, and fix it for PAH.

[bkontur]

@karolk91 Karol, can you please create a PR for this (not that prio), we can nicely reuse your benchmarking template setup for bench bot: #731 possibly, or maybe we could try some export-module-aliasing maybe in the main lib.rs?