Improve experiments registration/fetching API

[fo2rist]

Provide Cadabra instance and config as companion properties for consistency for both
Cadabra and CadabraAndroid
Allow registering experiments without Experiment object
Move Cadabra implementation to the separated file

Close https://github.com/fo2rist/cadabra/projects/1#card-22208695

[codecov]

Codecov Report

Merging #2 into master will increase coverage by 2.54%.
The diff coverage is 95.65%.

@@             Coverage Diff              @@
##             master       #2      +/-   ##
============================================
+ Coverage     93.75%   96.29%   +2.54%     
- Complexity        0       13      +13     
============================================
  Files             2        5       +3     
  Lines            16       27      +11     
  Branches          2        1       -1     
============================================
+ Hits             15       26      +11     
  Partials          1        1

Impacted Files	Coverage Δ	Complexity Δ
...om/github/fo2rist/cadabraandroid/CadabraAndroid.kt	100% <100%> (ø)	0 <0> (?)
...rc/main/java/com/github/fo2rist/cadabra/Cadabra.kt	100% <100%> (+8.33%)	0 <0> (ø)	⬇️
...main/java/com/github/fo2rist/cadabra/Experiment.kt	100% <100%> (ø)	0 <0> (ø)	⬇️
...ithub/fo2rist/cadabraandroid/CadabraAndroidImpl.kt	100% <100%> (ø)	1 <1> (?)
...ain/java/com/github/fo2rist/cadabra/CadabraImpl.kt	93.75% <93.75%> (ø)	12 <12> (?)

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