indicate whether hashrate estimation has converged

[plebhash]

with stratum-mining/sv2-apps#496:

• Pool + JDC: stable_hashrate is exposed on downstream SV2 channel data (/api/v1/clients/{id}/channels)
• tProxy: stable_hashrate is exposed on downstream SV1 client data (/api/v1/sv1/clients*)

here's some dirty mockups using emojis to illustrate how this could look

(not hard opinions, open to different approaches... also emojis probably not the best idea)

hashrate not stable:

hashrate stable: