refactor: improving request validation and client methods.

[vorobeyme]

• add a nil check to the Validate() method
• replace int64 with int in client methods and schemas
• replace bool with *bool in requests to allow null values
• update Patch() method (client) to handle different body types and perform validation
• improve null check in ListOptionsProvider
• move client schemas to the model package
• add helper functions etc.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 221 lines in your changes are missing coverage. Please review.

Project coverage is 2.49%. Comparing base (c025a0d) to head (d65c099).

Files	Patch %	Lines
crowdin/crowdin.go	0.00%	33 Missing ⚠️
crowdin/model/translations.go	0.00%	29 Missing ⚠️
crowdin/model/source_files.go	0.00%	19 Missing ⚠️
crowdin/model/source_strings.go	0.00%	18 Missing ⚠️
crowdin/projects.go	0.00%	15 Missing ⚠️
crowdin/model/string_translations.go	0.00%	14 Missing ⚠️
crowdin/model/errors.go	0.00%	13 Missing ⚠️
crowdin/model/projects.go	0.00%	12 Missing ⚠️
crowdin/model/branches.go	0.00%	9 Missing ⚠️
crowdin/source_files.go	0.00%	9 Missing ⚠️
... and 12 more

Additional details and impacted files

@@           Coverage Diff            @@
##            main     #14      +/-   ##
========================================
- Coverage   2.63%   2.49%   -0.14%     
========================================
  Files         22      23       +1     
  Lines        990    1046      +56     
========================================
  Hits          26      26              
- Misses       962    1018      +56     
  Partials       2       2              

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