fix: OPTIC-1785: Validate S3 bucket name on storage connection form

[mcanu]

This pull request introduces a mixin class to handle common functionality for S3 storage serializers and refactors existing serializers to use this mixin. The most important changes include the creation of the S3StorageSerializerMixin, the addition of a bucket name validation method, and the refactoring of S3ImportStorageSerializer and S3ExportStorageSerializer to use the new mixin.

Refactoring and code reuse:

• Created S3StorageSerializerMixin to encapsulate common functionality for S3 storage serializers, including secure field handling and bucket name validation.
• Refactored S3ImportStorageSerializer to inherit from S3StorageSerializerMixin and ImportStorageSerializer, consolidating common code and adding the presign field.
• Refactored S3ExportStorageSerializer to inherit from S3StorageSerializerMixin and ExportStorageSerializer, consolidating common code.

Validation improvements:

• Added validate_bucket method to S3StorageSerializerMixin to validate the bucket name using validate_bucket_name from botocore.handlers.

Code simplification:

• Replaced references to S3ImportStorageSerializer.secure_fields with self.secure_fields to simplify code and improve maintainability.

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[codecov]

Codecov Report

Attention: Patch coverage is 85.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.17%. Comparing base (4108f9e) to head (e3efe0c).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
label_studio/io_storages/s3/serializers.py	85.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7183      +/-   ##
===========================================
- Coverage    77.20%   77.17%   -0.04%     
===========================================
  Files          187      187              
  Lines        14484    14496      +12     
===========================================
+ Hits         11183    11187       +4     
- Misses        3301     3309       +8     

Flag	Coverage Δ
pytests	77.17% <85.00%> (-0.04%)	⬇️

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