Custom keyword modifying data doesn't run during parsing

Hi, I'm trying to parse to a custom type, but running into some issues.

**What version of Ajv are you using? Does the issue happen if you use the latest version?**
`"ajv": "^8.11.0"`

**Ajv options object**

```javascript
const parseDecimalKeyword: KeywordDefinition = {
keyword: "parseDecimal",
compile: () => (data, dataCtx: DataValidationCxt) => {
const parsedDecimal = new BigNumber(data, 10);
dataCtx.parentData[dataCtx.parentDataProperty] = parsedDecimal;

return !parsedDecimal.isNaN();
},
};

const ajv = new Ajv({
keywords: [parseDecimalKeyword],
});

```

**JSON Schema**

```javascript
{
elements: {
properties: {
amount: {
type: "string",
metadata: {
parseDecimal: true,
},
},
},
additionalProperties: true,
},
};

```

**Sample data**

```javascript
const test = [
{
id: 'DeQtD(X%t}',
creationTime: '2072-12-24T10:35:22.962Z',
description: '*N)`v1;#M8',
assetName: 'g+}rw`6zi)',
amount: '57136.58',
balance: '81053.34',
type: 'Withdraw',
recordType: 'Trade',
referenceId: '}AEcK*GK*P'
}
]

```

```javascript
const result = parseBtcMarketsTxsRspData(JSON.stringify(test))!;
expect(parseBtcMarketsTxsRspData.message).toBeUndefined();
expect(result).not.toBeUndefined();

expect(result).toHaveLength(1);
expect(result[0].amount).toBeInstanceOf(BigNumber);
```

**What results did you expect?**

Based on the examples in https://github.com/ajv-validator/ajv/issues/141 I expect to have the value of amount parsed to a BigNumber object if successful, but it seems like it's still a string.

Btw, it seems like I also have to cast the property object in schema to to satisfy the type checking even though I will be coercing the value from `string` to `BigNumber`. Is there a plan to address this limitation / support this use case?

**Are you going to resolve the issue?**

If there is a more appropriate way to do this, I'm happy to. Right now I have to work around it by using validate() instead of parse() directly.