Aurora serverless v2 support

[greedy52]

closes #13018

• Support AWS RDS Auora Serverless v2 #13018

No real code change is required. Only doc updates.

V2 is almost the same as standard Aurora except

• DB instance class is db.serverless
• Has some special serverless v2 scaling config.
• Engine mode is provisioned, not serverless

Testing

• URL format same as standard
• CA download, enable IAM auth all worked as expected.
• GRANT rds_iam TO alice; is still required.
• Tested tsh db ls/connect worked as expected

Some capture for reference.
aws rds describe-db-clusters

{
    "DBClusters": [
        {
            "EngineMode": "provisioned", 
            "Engine": "aurora-postgresql", 
            "Endpoint": "steve-postgres-serverless.cluster-c8gyaqgvclzt.ca-central-1.rds.amazonaws.com",
            "ReaderEndpoint": "steve-postgres-serverless.cluster-ro-c8gyaqgvclzt.ca-central-1.rds.amazonaws.com", 
            "IAMDatabaseAuthenticationEnabled": true, 
            ...
        }
    ]

aws rds describe-db-instances

{
    "DBInstances": [
        {
            "Engine": "aurora-postgresql", 
            "Endpoint": {
                "HostedZoneId": "Z1JG78A3UK1DU3", 
                "Port": 5432, 
                "Address": "steve-postgres-serverless-one.c8gyaqgvclzt.ca-central-1.rds.amazonaws.com"
            }, 
            "DBInstanceStatus": "available", 
            "IAMDatabaseAuthenticationEnabled": true, 
            "EngineVersion": "13.6", 
            "DBInstanceClass": "db.serverless", 
            ...
        }, 

[github-actions]

@greedy52 See the table below for backport results.

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