feat(select): add custom comparator input

[ascripcaru]

Please read and mark the following check list before creating a pull request:

• I read and followed the CONTRIBUTING.md guide.
• I read and followed the New Feature Checklist guide.

Short description of what this resolves:

Select options only worked when using primitives as values. If you had to use objects the === could not cope with the complex values. Thus, I added the possibility to add a custom compare function as a parameter for nebular select for more complex use cases and kept the old functionality in place.

Closes #2543. Closes #1223. Closes #1858.

[ascripcaru]

fixes issue #2543

[codecov]

Codecov Report

Merging #2590 (8bf8838) into master (09a7ba1) will decrease coverage by 0.00%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #2590      +/-   ##
==========================================
- Coverage   78.95%   78.94%   -0.01%     
==========================================
  Files         253      253              
  Lines        7815     7819       +4     
  Branches      857      857              
==========================================
+ Hits         6170     6173       +3     
- Misses       1382     1383       +1     
  Partials      263      263              

Impacted Files	Coverage Δ
...mework/theme/components/select/select.component.ts	88.46% <85.71%> (-0.20%)	⬇️
...endar-kit/services/calendar-month-model.service.ts	98.03% <0.00%> (ø)

[ghost]

DeepCode's analysis on #a58e55 found:

• ℹ️ 1 minor issue. 👇

Top issues

Description	Example fixes
Type declaration of 'any' loses type-safety. Consider replacing it with a more precise type. Occurrences: select.component.ts:56 select.component.ts:637 select.component.ts:668	🔧 Example fixes

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[ascripcaru]

hello @yggg. is there any chance for this functionality to be added in a next release of nebular?

[yggg]

Thanks @ascripcaru!