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 :warning:
crowdin/model/translations.go	0.00%	29 Missing :warning:
crowdin/model/source_files.go	0.00%	19 Missing :warning:
crowdin/model/source_strings.go	0.00%	18 Missing :warning:
crowdin/projects.go	0.00%	15 Missing :warning:
crowdin/model/string_translations.go	0.00%	14 Missing :warning:
crowdin/model/errors.go	0.00%	13 Missing :warning:
crowdin/model/projects.go	0.00%	12 Missing :warning:
crowdin/model/branches.go	0.00%	9 Missing :warning:
crowdin/source_files.go	0.00%	9 Missing :warning:
... and 12 more

Additional details and impacted files

```diff @@ 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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