statement-distribution: validator disabling (#1841)

Closes #1591.

The purpose of this PR is filter out backing statements from the network
signed by disabled validators. This is just an optimization, since we
will do filtering in the runtime in #1863 to avoid nodes to filter
garbage out at block production time.

- [x] Ensure it's ok to fiddle with the mask of manifests
- [x] Write more unit tests
- [x] Test locally
- [x] simple zombienet test
- [x] PRDoc

---------

Co-authored-by: Tsvetomir Dimitrov <tsvetomir@parity.io>

polkadot/node/network/statement-distribution/src/error.rs

@@ -75,6 +75,9 @@ pub enum Error {
 	#[error("Fetching availability cores failed {0:?}")]
 	FetchAvailabilityCores(RuntimeApiError),
 
+	#[error("Fetching disabled validators failed {0:?}")]
+	FetchDisabledValidators(runtime::Error),
+
 	#[error("Fetching validator groups failed {0:?}")]
 	FetchValidatorGroups(RuntimeApiError),