feat(docdb): support CA certificate for cluster instances (#28791)

Exposes the [CaCertificateIdentifier](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-docdb-dbinstance.html#cfn-docdb-dbinstance-cacertificateidentifier) property of [AWS::DocDB::DBInstance](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-docdb-dbinstance.html) in the L2 constructs `DatabaseCluster` and `DatabaseInstance` of `aws_docdb`. This allows specifying a custom CA identifier using the CaCertificate class.

Usage with `DatabaseCluster`:

```typescript
new DatabaseCluster(stack, 'Database', {
  // ...
  instanceType: InstanceType.of(InstanceClass.R5, InstanceSize.LARGE),
  instanceCaCertificate: CaCertificate.RDS_CA_RSA4096_G1,
  // ...
});
```

Usage with `DatabaseInstance`:

```typescript
new DatabaseInstance(stack, 'Instance', {
  cluster: databaseCluster,
  instanceType: InstanceType.of(InstanceClass.R5, InstanceSize.LARGE),
  caCertificate: CaCertificate.RDS_CA_RSA4096_G1,
});
```

This is modelled on #27138.

Closes #28356.

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