feat: add AWS Secrets Manager provider

[patrislav]

Summary

• Add aws SecretsProvider backed by AWS Secrets Manager, using AWS SDK Go v2
• Supports the default credential chain (env vars, ~/.aws/, IAM roles, EC2 instance profiles, etc.)
• Accepts optional config.LoadOptions for customization (e.g. region override)
• Add example usage in _examples/cmd/aws/

Usage

provider, err := aws.NewSecretsProvider(ctx)

// or with explicit region:
provider, err := aws.NewSecretsProvider(ctx, config.WithRegion("us-east-1"))

[marino39]

LGTM

[VojtechVitek]

LGTM