bench-bot: accept multiple runtimes and multiple chain types for Cumulus

[joao-paulo-parity]

Problem: The current syntax for Cumulus commands is

/cmd queue -c bench-bot $ pallet runtime chain_type pallet_name

which means that users are only allowed to specify one runtime and one chain_type per command.

While it's possible to benchmark multiple runtimes and chain_types through multiple commands, as demonstrated in paritytech/cumulus#1460 (comment), that is inconvenient since the first job which finishes will push a commit, therefore jobs which finish after it won't be able to push the commit because the HEAD of the branch has changed.

Solution: allow for specifying multiple runtimes and chain_types in a single command. For example:

/cmd queue -v -c bench-bot $ pallet statemine,westmint assets,contracts pallet_uniques

where multiple runtimes and chain_types can be concatenated by , or + or whatever.

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This feature might be interesting for Polkadot as well.