Allow justifications on non-finalized blocks

[nazar-pc]

Currently Substrate enforces that blocks must not have justifications unless they are finalized:

polkadot-sdk/substrate/client/service/src/client/client.rs

Line 606 in 1a38d6d

assert!(justifications.is_some() && finalized || justifications.is_none() || gap_block);

It is useful, however, for other purposes as well, for instance I would like to use justifications to make nodes send extra data alongside blocks that are not strictly required, but do help with block verification.

Grandpa already has a check that finalized blocks must have justifications (with redundant assertion afterwards) if I understand it correctly, so should not be an issue to remove this requirement (I think):

polkadot-sdk/substrate/client/consensus/grandpa/src/import.rs

Lines 548 to 555 in 1a38d6d

	if block.justifications.is_none() {
	return Err(ConsensusError::ClientImport(
	"Justification required when importing \
	an old block with authority set change."
	.into(),
	))
	}
	assert!(block.justifications.is_some());

Any concerns if I send a PR removing this restriction?

[bkchr]

CC @arkpar

[arkpar]

No objections from my side.
CC @andresilva

[bkchr]

Okay perfect :) @andresilva is on holidays. @nazar-pc let's create a pr :)