Implement custom rule to check spek test discovery performance issues

[arturbosch]

This PR introduces a new module custom-checks with for now one rule SpekTestDiscovery.
This rule finds variable declarations in spek tests where memoized would help to improve the performance.

All previous PR's improved the discovery time by ~200ms per module (500-1000ms in extreme cases like metrics and rules-style modules (parsing Kotlin files)).
In conclusion nearly every object we use to setup tests has to be considered as "heavy".
Exceptions are template strings and paths. I've excluded them from the rule.

Test frameworks like Spek which not only use reflection to find the tests but also need to instantiate a class and run the constructor closure are errorprone to such issues if one forgets to use memoized or similar concepts.

[codecov]

Codecov Report

Merging #2954 into master will decrease coverage by 0.35%.
The diff coverage is 44.87%.

@@             Coverage Diff              @@
##             master    #2954      +/-   ##
============================================
- Coverage     79.78%   79.42%   -0.36%     
- Complexity     2506     2517      +11     
============================================
  Files           427      430       +3     
  Lines          7534     7612      +78     
  Branches       1420     1437      +17     
============================================
+ Hits           6011     6046      +35     
- Misses          767      794      +27     
- Partials        756      772      +16     

Impacted Files	Coverage Δ	Complexity Δ
...ithub/detekt/custom/CustomChecksConfigValidator.kt	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...in/io/github/detekt/custom/CustomChecksProvider.kt	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...otlin/io/github/detekt/custom/SpekTestDiscovery.kt	64.81% <64.81%> (ø)	11.00 <11.00> (?)

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[arturbosch]

Rule works and all our tests are free of expensive constructs.
The rule needs type resolution so it will only run on detektTest etc

I will setup a CI job with type resolution next. For this other rule issues need to be fixed.