Add API to watchdog and skale-api to return only block health check for sChain

[sync-by-unito]

Currently watchdog (and skale-api) provides an ability to get only all sChain checks (config, DKG, RPC, blocks, volume, firewall, etc) which consumes lots of resources and time.
Proposal from validator: add an API to retrieve only the latest health check - blocks check, assuming that if blocks are mining and local RPC is available, then sChain is operating normally.

┆Issue is synchronized with this Jira Task