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>

substrate/primitives/weights/src/weight_meter.rs

@@ -149,6 +149,11 @@ impl WeightMeter {
 	pub fn can_consume(&self, w: Weight) -> bool {
 		self.consumed.checked_add(&w).map_or(false, |t| t.all_lte(self.limit))
 	}
+
+	/// Reclaim the given weight.
+	pub fn reclaim_proof_size(&mut self, s: u64) {
+		self.consumed.saturating_reduce(Weight::from_parts(0, s));
+	}
 }
 
 #[cfg(test)]
@@ -277,6 +282,21 @@ mod tests {
 		assert_eq!(meter.consumed(), Weight::from_parts(5, 10));
 	}
 
+	#[test]
+	#[cfg(debug_assertions)]
+	fn reclaim_works() {
+		let mut meter = WeightMeter::with_limit(Weight::from_parts(5, 10));
+
+		meter.consume(Weight::from_parts(5, 10));
+		assert_eq!(meter.consumed(), Weight::from_parts(5, 10));
+
+		meter.reclaim_proof_size(3);
+		assert_eq!(meter.consumed(), Weight::from_parts(5, 7));
+
+		meter.reclaim_proof_size(10);
+		assert_eq!(meter.consumed(), Weight::from_parts(5, 0));
+	}
+
 	#[test]
 	#[cfg(debug_assertions)]
 	#[should_panic(expected = "Weight counter overflow")]