PoV Reclaim Runtime Side (#3002)

# Runtime side for PoV Reclaim

## Implementation Overview
- Hostfunction to fetch the storage proof size has been added to the
PVF. It uses the size tracking recorder that was introduced in my
previous PR.
- Mechanisms to use the reclaim HostFunction have been introduced.
- 1. A SignedExtension that checks the node-reported proof size before
and after application of an extrinsic. Then it reclaims the difference.
- 2. A manual helper to make reclaiming easier when manual interaction
is required, for example in `on_idle` or other hooks.
- In order to utilize the manual reclaiming, I modified `WeightMeter` to
support the reduction of consumed weight, at least for storage proof
size.

## How to use
To enable the general functionality for a parachain:
1. Add the SignedExtension to your parachain runtime. 
2. Provide the HostFunction to the node
3. Enable proof recording during block import

## TODO
- [x] PRDoc

---------

Co-authored-by: Dmitry Markin <dmitry@markin.tech>
Co-authored-by: Davide Galassi <davxy@datawok.net>
Co-authored-by: Bastian Köcher <git@kchr.de>

cumulus/primitives/proof-size-hostfunction/src/lib.rs

@@ -18,6 +18,7 @@
 
 #![cfg_attr(not(feature = "std"), no_std)]
 
+#[cfg(feature = "std")]
 use sp_externalities::ExternalitiesExt;
 
 use sp_runtime_interface::runtime_interface;
@@ -35,7 +36,8 @@ pub const PROOF_RECORDING_DISABLED: u64 = u64::MAX;
 pub trait StorageProofSize {
 	/// Returns the current storage proof size.
 	fn storage_proof_size(&mut self) -> u64 {
-		self.extension::<ProofSizeExt>().map_or(u64::MAX, |e| e.storage_proof_size())
+		self.extension::<ProofSizeExt>()
+			.map_or(PROOF_RECORDING_DISABLED, |e| e.storage_proof_size())
 	}
 }