This PR provides the infrastructure for the pov-reclaim mechanism
discussed in #209. The goal is to provide the current proof size to the
runtime so it can be used to reclaim storage weight.

## New Host Function
- A new host function is provided
[here](https://github.com/skunert/polkadot-sdk/blob/5b317fda

/cumulus/primitives/pov-reclaim/src/lib.rs#L23).
It returns the size of the current proof size to the runtime. If
recording is not enabled, it returns 0.

## Implementation Overview
- Implement option to enable proof recording during import in the
client. This is currently enabled for `polkadot-parachain`,
`parachain-template` and the cumulus test node.
- Make the proof recorder ready for no-std. It was previously only
enabled for std environments, but we need to record the proof size in
`validate_block` too.
- Provide a recorder implementation that only the records the size of
incoming nodes and does not store the nodes itself.
- Fix benchmarks that were broken by async backing changes
- Provide new externalities extension that is registered by default if
proof recording is enabled.
- I think we should discuss the naming, pov-reclaim was more intuitive
to me, but we could also go with clawback like in the issue.

## Impact of proof recording during import
With proof recording: 6.3058 Kelem/s
Without proof recording: 6.3427 Kelem/s

The measured impact on the importing performance is quite low on my
machine using the block import benchmark. With proof recording I am
seeing a performance hit of 0.585%.

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Co-authored-by: command-bot <>
Co-authored-by: Davide Galassi <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>