Add support for setting on-demand throughput values for MaxWriteRequestUnits/MaxReadRequestUnits

[shlomil]

Feature described in here:
https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_OnDemandThroughput.html

[shlomil]

This worked for me:

@dataclass(frozen=True)
class MyOnDemandThroughput:
    max_read: int
    max_write: int

    def encode(self):
        return {
            "BillingMode": "PAY_PER_REQUEST",
            "OnDemandThroughput": {
                "MaxReadRequestUnits": self.max_read,
                "MaxWriteRequestUnits": self.max_write,
            }
        }