Code sample for sql.to()/from() should use secrets API

In this section: https://awesome.influxdata.com/docs/part-2/querying-and-data-transformations/#accessing-external-data-sources

The code for ```from()```:
```
sql.from(
driverName: "postgres",
dataSourceName: "postgresql://user:password@localhost",
query:"SELECT * FROM TestTable"
)
```
I suggest that we demonstrate using ```import "influxdata/influxdb/secrets"``` throughout.

Open the linked “Accessing External Data Sources” section and inspect the sql.from() example. Update the sample to demonstrate the secrets API throughout; it is done when the published example no longer embeds the username and password directly.

Written by the indexing model from the issue text.