Allow choosing a different parameter group (and option group) in the RDS read replica creation UI page

Question

Currently, the Create RDS read replica screen doesn't allow choosing a custom parameter group (and option group) on creation so
a new read replica with a smaller instance class ends up getting stuck in "incompatible-parameters" mode because of high values of buffer related parameters in the original primary instance's parameter group, so I have to create the new replica first with the original instance class of the primary instance, change the parameter group and then resize the replica to a smaller instance class.

Also, replication errors can occur while the primary parameter group is applied on the read replica because of MEMORY engine tables which have out of sync rows between the instances, which were meant to be excluded from replication using the read replica custom parameter group.

Answer

You can specify a parameter group using the api create-db-instance-read-replica

also, for memory parameters you can use formulas that are based on the size of the instance, this allows a common parameter group to work for any instance size:

```
            {
                "ParameterName": "innodb_buffer_pool_size",
                "ParameterValue": "{DBInstanceClassMemory*3/4}",
                "Description": "The size in bytes of the memory buffer innodb uses to cache data and indexes of its tables",
                "Source": "system",
                "ApplyType": "static",
                "DataType": "integer",
                "AllowedValues": "335544320-9223372036854775807",
                "IsModifiable": true
            },

Allow choosing a different parameter group (and option group) in the RDS read replica creation UI page

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