Implement average_duration for tasks and audits fields

[saberlynx]

What:

Why:

How:

Checklist:

• Tests
• CHANGELOG Entry
• Documentation

[codecov]

Codecov Report

Merging #14 (7b25a19) into master (f0255ab) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #14   +/-   ##
=======================================
  Coverage   97.07%   97.07%           
=======================================
  Files         121      121           
  Lines        7252     7258    +6     
=======================================
+ Hits         7040     7046    +6     
  Misses        212      212           

Impacted Files	Coverage Δ
selene/schema/audits/fields.py	93.33% <100.00%> (+0.35%)	⬆️
selene/schema/tasks/fields.py	98.24% <100.00%> (+0.03%)	⬆️

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