Add custom widgets to range slider

[deandreamatias]

Connected to #1148

Solution description

Add properties to allow customize widgets to show max, min and current value on range slider form field

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 #1224 (01bc243) into main (71cf8db) will decrease coverage by 0.10%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##             main    #1224      +/-   ##
==========================================
- Coverage   83.75%   83.66%   -0.10%     
==========================================
  Files          19       19              
  Lines         708      710       +2     
==========================================
+ Hits          593      594       +1     
- Misses        115      116       +1     

Impacted Files	Coverage Δ
lib/src/fields/form_builder_range_slider.dart	94.28% <87.50%> (-2.69%)	⬇️

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