JSON API: allow more query parameters

`wpcom_json_api_request_as_blog()` is currently used by Jetpack itself, but can also be used by third-party plugin authors via functions like `stats_get_from_restapi()`.

Unfortunately, one cannot make custom API queries via `stats_get_from_restapi()` right now, because the query parameters are filtered out here:
https://github.com/Automattic/jetpack/blob/4.4.2/class.jetpack-client.php#L273

Would it be possible to allow more parameters, so one can do requests like this for example:

```php
// Retrieve the 50 more popular posts every week in the past 7 weeks.
$args = array(
'num' => 7,
'period' => 'week',
'max' => 50,
);
$stats = stats_get_from_restapi( $args, 'top-posts' );
```

Reported in 2872802-t

Start in class.jetpack-client.php at the linked filtering code around line 273, then trace how wpcom_json_api_request_as_blog() and stats_get_from_restapi() pass query arguments. Confirm the filtering prevents the example parameters from reaching the JSON API; done means custom parameters such as num, period, and max are accepted without removing existing safeguards.

Written by the indexing model from the issue text.