all.equal.data.table: sub-aggregate before comparison

[venom1204]

closes #7890

this pr adds by and fun.aggregate (defaulting to sum) arguments to all.equal.data.table.
this allows sub aggregation to align datasets at different levels of granularity before performing equality checks.

hi @tdhock @joshhwuu can you have a look when u got time , thanks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.02%. Comparing base (e26cf1b) to head (e75d652).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7894   +/-   ##
=======================================
  Coverage   99.02%   99.02%           
=======================================
  Files          88       88           
  Lines       17376    17381    +5     
=======================================
+ Hits        17206    17211    +5     
  Misses        170      170           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.