`channels_sv2::server` should expose whether `vardiff` has converged (via `stable_hashrate: bool` statevar)

[plebhash]

channels_sv2::vardiff::classic::VardiffState::try_vardiff returns:

• Ok(Some(new_hashrate)) when an update occurs
• Ok(None) when conditions don’t warrant an update
• Err for actual processing errors

we need to add the following methods to channels_sv2::server::{standard,extended}:

• set_stable_hashrate(bool: stable_hashrate)
• get_stable_hashrate() -> bool

the application layer would be responsible for calling these on its try_vardiff tasks, by following this logic:

match try_vardiff(...) {
  Ok(Some(new_hashrate)) => channel.set_stable_hashrate(false),
  Ok(None) => channel.set_stable_hashrate(true),
  Err(_) => {},
}

that way, stratum-apps can poll get_stable_hashrate to expose HTTP APIs that allow sv2-ui to display informations as suggested by @average-gary :

note: I don't think stratum-mining/sv2-apps#396 has much to do with this

at best, making vardiff cycles shorter (which is not a low-hanging fruit on sv2-apps) only would make the proposed vardiff progress indicator converge to ✅ a bit faster, which is good but not the point to be unblocked here