Add support for proc resource in one polymorphic associations

[alejandroperea]

This PR adds support for using procs in one associations for selecting the resource serializer.

So, now will be able to use something like this:

  class SoundtrackSerializer
    include Alba::Resource

    one :content, resource: ->(content) { content.is_a?(Movie) ? MovieResource : TvShowResource }
  end

[codecov]

Codecov Report

Merging #281 (6316509) into main (b7ddbeb) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #281   +/-   ##
=======================================
  Coverage   99.18%   99.18%           
=======================================
  Files          12       12           
  Lines         488      489    +1     
=======================================
+ Hits          484      485    +1     
  Misses          4        4           

Impacted Files	Coverage Δ
lib/alba/association.rb	100.00% <100.00%> (ø)

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[okuramasafumi]

Thank you!