feat: add custom widgets to legend slider

[deandreamatias]

Connection with issue(s)

Close #1207

Solution description

• Add custom widgets to slider
• Add dart docs

To Do

• Read contributing guide
• Check the original issue to confirm it is fully satisfied
• Add solution description to help guide reviewers
• Add unit test to verify new or fixed behaviour
• If apply, add documentation to code properties and package readme

[codecov]

Codecov Report

Merging #1233 (53d9ab6) into main (76a1074) will decrease coverage by 0.13%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##             main    #1233      +/-   ##
==========================================
- Coverage   86.15%   86.03%   -0.13%     
==========================================
  Files          20       20              
  Lines         715      716       +1     
==========================================
  Hits          616      616              
- Misses         99      100       +1     

Impacted Files	Coverage Δ
lib/src/fields/form_builder_range_slider.dart	97.05% <ø> (ø)
lib/src/fields/form_builder_slider.dart	97.05% <85.71%> (-2.95%)	⬇️

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