✨ add fill_value option to gap handlers

[jonasvdd]

This PR add the fill_value option to the AbstractGapHandler interface, which allows to conveniently fill gaps of traces, that utilize shaded areas via a fill="tozeory".

TODO:

• update docs for area's and fill-values

[codecov-commenter]

Codecov Report

Merging #218 (ba9dd95) into main (ef0d4e9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##             main     #218   +/-   ##
=======================================
  Coverage   97.16%   97.17%           
=======================================
  Files          13       13           
  Lines         989      991    +2     
=======================================
+ Hits          961      963    +2     
  Misses         28       28           

Impacted Files	Coverage Δ
...tly_resampler/aggregation/gap_handler_interface.py	95.45% <100.00%> (+0.45%)	⬆️
..._resampler/aggregation/plotly_aggregator_parser.py	94.56% <100.00%> (ø)

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[jvdd]

LGTM