fix: Do not assemble a block larger than blobs allowance

[spalladino]

The sequencer publisher enforces a max size for a block, depending on the size it takes up in blobs. If the block exceeds that size, it's rejected by the publisher.

aztec-packages/yarn-project/stdlib/src/block/body.ts

Lines 56 to 70 in bb87ea4

	toBlobFields() {
	let flattened: Fr[] = [];
	this.txEffects.forEach((effect: TxEffect) => {
	flattened = flattened.concat(effect.toBlobFields());
	});
	if (flattened.length > BLOBS_PER_BLOCK * FIELDS_PER_BLOB) {
	throw new Error(
	`Attempted to overfill block's blobs with ${flattened.length} elements. The maximum is ${
	BLOBS_PER_BLOCK * FIELDS_PER_BLOB
	}`,
	);
	}
	return flattened;
	}

This PR adds a check during block building to ensure that we don't go past that limit.