[Mapper] Add `ignore_null` option to `timestamp`

Related to #9049.

By default, the default value for `timestamp` is `now` which means the date the document was processed by the indexing chain.

You can now reject documents which not provide a `timestamp` value by setting `ignore_null` to false (default to `true`):

```js
{
    "tweet" : {
        "_timestamp" : {
            "enabled" : true,
            "ignore_null" : false
        }
    }
}
```

Closes #8882.