statement-distribution: elastic scaling support

[sandreim]

The current implementation assumes a 1:1 mapping between a parachain and backing group/core in quite a few places:

• polkadot-sdk/polkadot/node/network/statement-distribution/src/v2/mod.rs

  Line 2129 in 93b1abb

  fn group_for_para(

• polkadot-sdk/polkadot/node/network/statement-distribution/src/v2/mod.rs

  Line 2195 in 93b1abb

  let group_index = group_for_para(

• polkadot-sdk/polkadot/node/network/statement-distribution/src/v2/mod.rs

  Line 2314 in 93b1abb

  let expected_group = group_for_para(

• polkadot-sdk/polkadot/node/network/statement-distribution/src/v2/mod.rs

  Line 3040 in 93b1abb

  let expected_group = group_for_para(

To fix this we need to implement something similar to the assumed core index in the backing subsystem. This would then enable the subsystem to properly distribute/filter manifests based on the validator index / expected group considering there can be more than 1 backing group for a certain parachain.