[Connectors-SDK] Handle config deprecation and migration in connectors-sdk

[throuxel]

Proposed changes

This PR introduces a deprecation handling system for the connectors SDK, managing automatically different use-cases while maintaining backward compatibility.

• LegacyField: Mark fields as deprecated with migration metadata
• Automatic migration via migrate_deprecation() validator in BaseConnectorSettings
• Support for variable renaming, namespace migration, and value transformation
• User-friendly deprecation warnings
• Compatible with the connector composer
• Tests added

Exemple:

class MyConfig(BaseConfigModel):
   old_var: SkipValidation[int] = LegacyField(
       deprecated="Use new_var instead",
       new_variable_name="new_var",
       change_value=lambda x: x * 60,  # Optional transformation
       removal_date="2026-09-01" # Optional informative date
   )
   new_var: int = Field(description="New variable")

class ConnectorSettings(BaseConnectorSettings):
   old_namespace: SkipValidation[MyConfig] = LegacyField(
       deprecated="Use new_namespace instead",
       new_namespace="new_namespace"
   )
   new_namespace: MyConfig = Field(default_factory=MyConfig)

Related issues

• Closes [Connectors-SDK] Handle config deprecation and migration in connectors-sdk #5859

Checklist

• I consider the submitted work as finished
• I have signed my commits using GPG key.
• I tested the code for its functionality using different use cases
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

The dummy connector is only for demo, and should be removed before merging.

[Powlinett]

Thank you so much for this great contribution @throuxel 🤘

I don't approve the PR right now to leave room for discussion, but as far as I tested it works as expected ✅️

Most of my comments are about naming things and it's obviously debatable.

Plus, I noticed 10 new warnings emitted while running tests, and I couldn't find the actual cause:

tests/test_settings/test_base_settings.py: 8 warnings
tests/test_settings/test_deprecation_migration.py: 2 warnings
  ~/code/filigran/connectors/connectors-sdk/venv/lib/python3.12/site-packages/pydantic_settings/main.py:447: UserWarning: Config key `yaml_file` is set in model_config but will be ignored because no YamlConfigSettingsSource source is configured. To use this config key, add a YamlConfigSettingsSource source to the settings sources via the settings_customise_sources hook.
    cls._settings_warn_unused_config_keys(sources, cls.model_config)

Do you have any idea of what's happening?

Thanks again, well done 💪

[Powlinett]

LGTM 👏 let us know when you're ready for merge 😇

[jabesq]

LGTM 👍