core: rename fetch args and update docs

[corverroos]

Renames FetchArgs to DutyDefinition and update docs.

category: refactor
ticket: #698

[codecov]

Codecov Report

Merging #717 (c2448bf) into main (62b3975) will increase coverage by 0.09%.
The diff coverage is 74.41%.

@@            Coverage Diff             @@
##             main     #717      +/-   ##
==========================================
+ Coverage   53.97%   54.07%   +0.09%     
==========================================
  Files         104      104              
  Lines       10162    10162              
==========================================
+ Hits         5485     5495      +10     
+ Misses       3871     3867       -4     
+ Partials      806      800       -6     

Impacted Files	Coverage Δ
core/dutydefinition.go	0.00% <0.00%> (ø)
core/interfaces.go	0.00% <0.00%> (ø)
core/retry.go	0.00% <0.00%> (ø)
core/tracing.go	15.78% <0.00%> (ø)
core/types.go	20.40% <0.00%> (ø)
core/validatorapi/validatorapi.go	50.87% <80.00%> (ø)
core/scheduler/scheduler.go	73.87% <94.73%> (ø)
app/app.go	59.25% <100.00%> (+1.50%)	⬆️
core/fetcher/fetcher.go	65.21% <100.00%> (ø)
core/scheduler/metrics.go	100.00% <100.00%> (ø)
... and 5 more

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