[Feature request] Unnest array support

Hi! We use BigQuery and work with arrays.

We try to define models such as:

```
cube('a', {
sql: `unnest(metrics)`,
...
});
```

```
cube('b', {
joins: {
a: {
relationship: 'one_to_many',
sql: `(true)`
}
},
...
});
```

The generated SQL looks like:

```LEFT JOIN (unnest(metrics)) AS a ON (true)```

and BigQuery throws an error:

```Expected keyword JOIN but got ")" at [5:37]```

Feature request: Add a check if the SQL is unnest(..) and do not wrap it with additional brackets.