feat: estimate time by offset

[mujahidkay]

Description

Closes: #5

This PR adds an -offset arg to the estimator to more accurately estimate the expected time of a future block

Without offset

❯ go run main.go -height 17842500 -rpc https://main-a.rpc.agoric.net:443 -samples 1000
Mean Block Time: 14.825253s (1000 samples)
Estimated time for height 17842500:
        Local: Sat Jan 11 14:57:16 PKT 2025
          UTC: Sat Jan 11 09:57:16 UTC 2025
   Asia/Tokyo: Sat Jan 11 18:57:16 JST 2025
Australia/NSW: Sat Jan 11 20:57:16 AEDT 2025
Asia/Istanbul: Sat Jan 11 12:57:16 +03 2025
   US/Pacific: Sat Jan 11 01:57:16 PST 2025
   US/Eastern: Sat Jan 11 04:57:16 EST 2025
****** WARNING ******
Insufficient time for voting period: 70h5m55.450267553s
****** WARNING ******

With offset

❯ go run main.go -height 17842500 -offset 7.2d  -rpc https://main-a.rpc.agoric.net:443 -samples 1000
Mean Block Time: 14.762233s (1000 samples)
Estimated time for height 17842500:
        Local: Fri Jan 10 19:59:43 PKT 2025
          UTC: Fri Jan 10 14:59:43 UTC 2025
   Asia/Tokyo: Fri Jan 10 23:59:43 JST 2025
Australia/NSW: Sat Jan 11 01:59:43 AEDT 2025
Asia/Istanbul: Fri Jan 10 17:59:43 +03 2025
   US/Pacific: Fri Jan 10 06:59:43 PST 2025
   US/Eastern: Fri Jan 10 09:59:43 EST 2025
****** WARNING ******
Insufficient time for voting period: 51h9m11.308282955s
****** WARNING ******

Since we were targeting Friday 10th 14 UTC, the latter seems more accurate than the former. Since I did not have any other data points, so this is the only verification I did.