[CI] Benchmark multiple runtimes at once

[Dinonard]

Description

Running benchmarks only allows user to specify one runtime at a time.
This is problematic if we want to benchmark all of them at once, since each trigger
will rebuild the entire binary which takes time.

To improve this, suggestion is to change

# [chain name] - astar-dev, shiden-dev, shibuya-dev, dev
# [pallet names] - Use "," for multiple pallets, "all" for all pallets
/bench [chain name] [pallet names]

into

# [chain names] - Use "," for multiple runtimes. Available values are: astar-dev, shiden-dev, shibuya-dev, dev
# [pallet names] - Use "," for multiple pallets, "all" for all pallets
/bench [chain names] [pallet names]

The results should ideally be grouped in folders, according to their runtime.