Allow for indexer to iterate backwards from chain head

[deekerno]

Currently, all indexers start from the genesis block and iterate forwards to the head of the chain. Once completed, this provides a complete history of a contract as it pertains to one's indexer. This is helpful when the recency of indexed data is the same across time or having the most recent data right now isn't of the highest importance; an example of this could be indexing data for historical audits.

However, there may be use cases in which recent indexed data is more important than older historical data; for example, a dApp may want to index the most recent data first as their users are more likely to query for information in the last few blocks rather than the first few blocks of the chain.

This could be as simple as adding something akin to a direction field to the manifest and then adjusting the query to the FuelGqlClient accordingly.

[0xmovses]

This issue is related to #796 . I think it would be prudent to get #796 merged first then move onto this.