Able to chill inactive validator

[aurexav]

Add a new extrinsic chill_inactive_validator and a new storage config ChillInactiveValidatorThreshold:

• Anyone can call this function.
• The validator's stash account should be provided.
• The proof should be a list of consecutive sorted era indexes where the validator has
  not produced any blocks.
• This will only be successful if the validator has not produced any blocks in ChillInactiveValidatorThreshold consecutive eras.
• If the ChillInactiveValidatorThreshold is not set, it will fallback to Config::HistoryDepth. It's almost the same as current on-chain logic. The inactive validator is almost impossible to be chilled by others, unless it missed 84 eras.

Closes #5674.

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Polkadot address: 156HGo9setPcU2qhFMVWLkcmtCEGySLwNqa3DaEiYSWtte4Y