Update /extended/v1/status to "syncing" if more than X blocks are missing

[mardi00]

Is your feature request related to a problem? Please describe.
On the /extended/v1/status endpoint, the status is 'ready' even if a hundred blocks still need to be synced
{ "server_version": "stacks-blockchain-api v2.0.0 (master:8b95f1d9)", "status": "ready" }

Describe the solution you'd like
When more than X blocks (5, for example) still need to be synced, we would have the 'syncing' status
{ "server_version": "stacks-blockchain-api v2.0.0 (master:8b95f1d9)", "status": "syncing" }

[zone117x]

Related to existing issue #698

The API does not have the information required for this feature. Once PR stacks-network/stacks-core#2768 exposes sync status / block percentage, then the API could potentially expose that info as well. Although, if an RPC endpoint already exposes it, there's no real reason for the API to duplicate that in another endpoint.

[sabbyanandan]

Given the upstream's RPC endpoint already supports it, let's not duplicate this. Closing this for now.