feat: [Indexer] check indexer rediness

Before returning data on http api - indexer need to check whether it is fully/almost synced to returned data. Otherwise should return appropriate error.

Start at the HTTP API entry points that return indexer data and trace how the indexer's sync state is exposed. Determine the appropriate readiness condition and error response from the surrounding API behavior. Done means requests do not return data until the indexer is fully or almost synced, and return the agreed error otherwise.

Written by the indexing model from the issue text.