feat: add AWS Secrets Manager secret operations

[atharva-nagane]

Context

RustCloud covers AWS security via IAM (identity) and KMS (key management)
but has no support for AWS Secrets Manager — the service for storing and
retrieving application secrets (API keys, passwords, tokens).

This completes the AWS security category: IAM controls who can access what,
KMS encrypts keys, Secrets Manager stores the actual secret values.

Operations to implement

Function	Description
create_secret	Store a new plaintext or binary secret
get_secret	Retrieve secret string or binary by name/ARN
update_secret	Rotate or update a secret's value
delete_secret	Schedule deletion (with optional recovery window)
list_secrets	List all secrets in the account

Acceptance criteria

• All 5 functions following the existing SNS flat-function pattern
• aws-sdk-secretsmanager added to Cargo.toml
• Module wired in main.rs and tests/mod.rs
• 5 tests added
• Example doc at examples/aws/security/secrets_manager.md
• README updated (Security row)

Files

• rustcloud/src/aws/aws_apis/security/aws_secrets_manager.rs ← new
• rustcloud/src/tests/aws_secrets_manager_operations.rs ← new
• examples/aws/security/secrets_manager.md ← new
• rustcloud/Cargo.toml ← add dep
• rustcloud/src/main.rs ← wire module
• rustcloud/src/tests/mod.rs ← wire tests
• README.md ← update Security row