PoV Reclaim (Clawback) Node Side (#1462)

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/5b317fda3be205f4136f10d4490387ccd4f9765d/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%.

---------

Co-authored-by: command-bot <>
Co-authored-by: Davide Galassi <davxy@datawok.net>
Co-authored-by: Bastian Köcher <git@kchr.de>

substrate/client/api/src/execution_extensions.rs

@@ -91,7 +91,6 @@ impl<Block: BlockT, Ext: Default + Extension> ExtensionsFactory<Block>
 ///
 /// This crate aggregates extensions available for the offchain calls
 /// and is responsible for producing a correct `Extensions` object.
-/// for each call, based on required `Capabilities`.
 pub struct ExecutionExtensions<Block: BlockT> {
 	extensions_factory: RwLock<Box<dyn ExtensionsFactory<Block>>>,
 	read_runtime_version: Arc<dyn ReadRuntimeVersion>,
@@ -116,8 +115,7 @@ impl<Block: BlockT> ExecutionExtensions<Block> {
 		*self.extensions_factory.write() = Box::new(maker);
 	}
 
-	/// Based on the execution context and capabilities it produces
-	/// the extensions object to support desired set of APIs.
+	/// Produces default extensions based on the input parameters.
 	pub fn extensions(
 		&self,
 		block_hash: Block::Hash,
@@ -127,7 +125,6 @@ impl<Block: BlockT> ExecutionExtensions<Block> {
 			self.extensions_factory.read().extensions_for(block_hash, block_number);
 
 		extensions.register(ReadRuntimeVersionExt::new(self.read_runtime_version.clone()));
-
 		extensions
 	}
 }