Add design doc for system user password rotation

Introduce a CredentialRotation CRD with a dedicated controller to
rotate all 8 system user passwords without MySQL downtime, using
MySQL's dual password feature (8.0.14+).

Key design points:
- Dedicated CRD isolates rotation from MySQLClusterReconciler
- Two-step process (rotate then discard) with verification window
- spec.rotationGeneration counter for GitOps/ArgoCD compatibility
- Crash-safe and idempotent at every phase boundary

Signed-off-by: shunki-fujita <shunki-fujita@cybozu.co.jp>

Author: shunki-fujita