Get all available block count via RPC

[ZenVoich]

Is your feature request related to a problem? Please describe.
I want to show my own node syncing progress bar, but I could not find a way to get all available block count via RPC call.

Describe the solution you'd like
Maybe add availableHeaderCount or totalHeaderCount to getBlockDagInfo result

Describe alternatives you've considered
getBlockDagInfo returns headerCount - synced headers, and blockCount - synced blocks, but no way to get not yet synced total header count. So if I use them it shows wrong results until headers are synced.

Additional context
I see IBD: Processed 39960 block headers (60%) in node output. I suppose that number is available internally