Add snapshot restore support for AWS::RedshiftServerless::Namespace

### Name of the resource

AWS::RedshiftServerless::Namespace

### Resource name

_No response_

### Description

## Summary
AWS::RedshiftServerless::Namespace currently does not support snapshot restore functionality via CloudFormation, forcing users to rely on manual AWS CLI operations for disaster recovery and environment promotion scenarios.

## Service
Amazon Redshift Serverless

## Resource Type
AWS::RedshiftServerless::Namespace

## Current Limitation
The `AWS::RedshiftServerless::Namespace` resource type lacks properties to restore from existing snapshots. Currently, only new namespace creation is supported.

## Requested Properties
Add the following properties to `AWS::RedshiftServerless::Namespace`:

- `SnapshotName` (String, Optional) - Name of the snapshot to restore from (same account)
- `SnapshotArn` (String, Optional) - Full ARN of the snapshot to restore from (cross-account support)

When either property is specified:
- `DbName`, `AdminUsername`, and `AdminUserPassword` should be optional/ignored (restored from snapshot)
- The namespace should be created from the specified snapshot instead of as a new empty namespace

## Use Cases
1. **Disaster Recovery**: Restore production data to a new environment from snapshots
2. **Environment Promotion**: Copy data from prod to staging/dev environments
3. **Cross-Account Data Sharing**: Restore snapshots shared from other AWS accounts
4. **Infrastructure as Code**: Manage complete Redshift Serverless lifecycle via CloudFormation

## Current Workaround
Manual AWS CLI operations:
```bash
aws redshift-serverless restore-from-snapshot \
--namespace-name "my-namespace" \
--snapshot-arn "arn:aws:redshift-serverless:us-east-1:123456789012:snapshot/snapshot-id"

### Other Details

_No response_

No repository files or tests are identified. Start by reviewing the AWS::RedshiftServerless::Namespace resource and the documented restore-from-snapshot CLI operation. Done means supporting SnapshotName and SnapshotArn, restoring the namespace from the selected snapshot, and making DbName, AdminUsername, and AdminUserPassword optional or ignored when restoring.

Written by the indexing model from the issue text.