[DataAvailability] Allow syncing Execution Data before block is sealed

[peterargue]

Problem Definition

Currently, the ExecutionDataRequester waits until a block is sealed before it begins to download its BlockExecutionData. This simplified the initial implementation at the expense of added latency. Now that execution data is indexed and the data is used to serve the Access API, there's greater need to reduce that latency. A few of the main reasons:

• Currently about 30% of script executions on mainnet are for blocks that have not yet been indexed by the AN. Reducing the latency would allow ANs to serve an even higher percent.
• As data sizes get larger (more tx/events, larger account storage, etc) it will take longer to download the data. Starting earlier will ensure it's available within a reasonable amount of time.
• Some users want access to data as soon as the block is executed. By syncing before sealed, the data will become available several seconds sooner and likely within a tolerable range for these users.
• Eventually, we will want to phase out the use of Execution nodes to service requests from Access nodes for the Access API.

Proposed Solution

Update the requester to sync execution data for blocks that are executed but not yet sealed. There is an initial implementation that we can use as a starting point: #2842. This may also work as-is. We originally did not deploy this to keep the system simpler during it's initial rollout.

Staked access nodes subscribe to the ExecutionReceipts channel which is where Execution nodes publish notifications for newly executed blocks. These block may or may not be finalized yet.

Definition of Done

• BlockExecutionData is downloaded by Access nodes before the block is sealed. Ideally as soon after the block was executed as possible.
• Data from orphaned forks is pruned from the db.
• There are metrics and logging that provide observibility into how the system is performing, and the latency.
• There is unit and integration test coverage for the new features.
• There is documentation both in code and a readme outlining how the system works.