Automatically retrieve Redshift cluster credentials

**Is your feature request related to a problem? Please describe.**
The AWS Redshift SDK allows retrieving credentials via an API:

```typescript
import Redshift, { ClusterCredentials } from 'aws-sdk/clients/redshift';

const getCredentials = async () => {
const creds = await redshift.getClusterCredentials({
ClusterIdentifier: process.env.CUBEJS_DB_HOST,
DbUser: process.env.CUBEJS_DB_USER,
}).promise();

if (!creds.DbUser && !creds.DbPassword) {
throw new Error('Unable to retrieve Redshift credentials');
}

return creds as Required;
};

getCredentials()
.then((credentials) => {
const config = {
database : process.env.CUBEJS_DB_NAME,
host: process.env.CUBEJS_DB_HOST,
password : creds.DbPassword,
port: process.env.CUBEJS_DB_PORT,
user: creds.DbUser,
};
})
```

**Describe the solution you'd like**

It would be great if CubeJS would automatically attempt to do the above if the `CUBEJS_DB_PASS` environment variable is omitted for Redshift projects.

**Additional context**
A Serverless project using Redshift would require additional permissions to the IAM role:

```yaml
iamRoleStatements:
- Effect: 'Allow'
Action:
- 'redshift:GetClusterCredentials'
Resource:
- '/'
```