feat: Cost tracker for CostTriggers

[robinholzi]

Motivation

We want to use a linear model ($T(D) :=aD+b$, $T :=$ time, $D :=$ samples) to estimate training costs/time for our cost-aware policies.

CostTracker keeps track of previous training costs and provides the model prediction functionality.

[github-actions]

^{( % to main)
( % to main)}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.29%. Comparing base (364a56a) to head (33f7ce6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #599      +/-   ##
==========================================
+ Coverage   85.26%   85.29%   +0.02%     
==========================================
  Files         248      249       +1     
  Lines       10991    11009      +18     
==========================================
+ Hits         9372     9390      +18     
  Misses       1619     1619              

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[MaxiBoether]

Thanks :)