Add SecretMarkingProvider for post-hoc secret marking

I'd like to propose adding `SecretMarkingProvider` - a wrapper provider that marks configuration values as secrets based on key patterns.

## Use case

When integrating with external configuration sources (environment variables, third-party providers) that don't properly identify sensitive data, users need a way to retroactively mark values as secrets.

## Current workaround

Implement a custom provider wrapper manually, duplicating boilerplate code.

## Proposed solution

A new `SecretMarkingProvider` wrapper that:
- Wraps any existing `ConfigProvider`
- Marks values as secrets when keys match a user-provided predicate
- Preserves existing secret status (never removes `isSecret` flag)

```swift
let envProvider = EnvironmentVariablesProvider()
let secretMarkedProvider = envProvider.markSecrets { key in
key.description.contains("password")
}
```

I have a working implementation and proposal (SCO-0004) ready for review.

**Implementation branch:** https://github.com/gautamrajur/swift-configuration/tree/feature/secret-marking-provider

Review proposal SCO-0004 and the feature/secret-marking-provider branch first, since the issue says a working implementation already exists there. Check that the provider wraps any ConfigProvider, applies the user predicate to keys, and preserves existing secret status; done means the proposed API and behavior are ready for review.

Written by the indexing model from the issue text.