Add an option to pass additional current_setting variable to PostgresDriver

Need to pass some variable to PostgreSQL current_setting to fetch store specific data in multi-tenant application. On checking PostgresDriver package code, there is an option to SET current settings variable as following

```
async prepareConnection(conn, options = {
executionTimeout: this.config.executionTimeout ? (this.config.executionTimeout) * 1000 : 600000
}) {
await conn.query(`SET TIME ZONE '${this.config.storeTimezone || 'UTC'}'`);
await conn.query(`SET statement_timeout TO ${options.executionTimeout}`);
await this.loadUserDefinedTypes(conn);
}
```

I need to set one(or more) additional current settings variable, so that my view will return current store specific data only.

**Describe the solution you'd like**
I need an option to pass some additional variables and it should loop and set current settings before running the cube queries.

PostgresDriver Initialization

```
new PostgresDriver({
database: process.env.CUBEJS_DEMOAG_DB_NAME,
host: process.env.CUBEJS_DEMOAG_DB_HOST,
user: process.env.CUBEJS_DB_USER,
password: process.env.CUBEJS_DB_PASS,
port: process.env.CUBEJS_DB_PORT,
options: {
storeid: 'store-1',
role: 'admin'
}
})
```
prepareConnection function should loop options given and set variable accordingly.

```
async prepareConnection(conn, options = {
executionTimeout: this.config.executionTimeout ? (this.config.executionTimeout) * 1000 : 600000
}) {
await conn.query(`SET TIME ZONE '${this.config.storeTimezone || 'UTC'}'`);
await conn.query(`SET statement_timeout TO ${options.executionTimeout}`);

**await conn.query(`SET ${given_options.variable_name} = ${given_options.value}`);**

await this.loadUserDefinedTypes(conn);
}
```