Add `NestedSecretsSettingsSource`

[makukha]

The current implementation of SecretsSettingsSource does not work for nested models. This appears to be a pretty common use case:

• GitHub issue: BaseSettings fails to populate nested module with loaded secrets #154
• PyPI package: pydantic-file-secrets

To maintain backwards compatibility within pydantic-settings v2, a separate settings source can be ported from pydantic-file-secrets package.

[makukha]

@hramezani are you ok with the NestedSecretsSettingsSource name?

[hramezani]

@hramezani are you ok with the NestedSecretsSettingsSource name?

Yes, looks good