feat: overwrite global setting with individual DAG config

[yottahmd]

Allow users to choose whether or not to send mail on failure with individual DAGs by overriding the MailOn settings.

[codecov-commenter]

Codecov Report

Merging #201 (44be488) into main (033f467) will increase coverage by 0.27%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #201      +/-   ##
==========================================
+ Coverage   85.08%   85.36%   +0.27%     
==========================================
  Files          46       46              
  Lines        3239     3253      +14     
==========================================
+ Hits         2756     2777      +21     
+ Misses        353      346       -7     
  Partials      130      130              

Impacted Files	Coverage Δ
internal/config/config.go	84.47% <100.00%> (+0.16%)	⬆️
internal/config/loader.go	85.51% <100.00%> (+1.07%)	⬆️
internal/reporter/reporter.go	97.43% <100.00%> (ø)
agent.go	80.71% <0.00%> (+2.50%)	⬆️

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