[Bug]: No CashInterest Model for CreateAccountRequest

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Current Behavior

According to the [Create an Account](https://docs.alpaca.markets/reference/createaccount) documentation there is a `cash_interest` object available, however it is missing from this SDK.

### Expected Behavior

There should be a `CashInterest` model available on the SDK that can be used with `CreateAccountRequest`

### SDK Version I encountered this issue in

alpaca-py version: 0.43.2

### Steps To Reproduce

```markdown
1. Install alpaca-py version 0.43.2
2. Attempt to import the CashInterest model
3. Receive an error that the model does not exist
```

### Filled out the Steps to Reproduce section?

- [x] I have entered valid steps to reproduce my issue or have attached a minimally reproducible case in code that shows my issue happening; and understand that without this my issue will be flagged as invalid and closed after 30 days.

### Anything else?

_No response_