Add support for Posit Connect service account flow with Databricks using `connectcreds`

[tnederlof]

There is already a helper function that detects when "Viewer" type credentials exist here:

odbc/R/driver-databricks.R

Line 219 in 258b74a

# Detect viewer-based credentials from Posit Connect.

It would be helpful for those using "Service Account" style credentials to be able to use the same helper function. It looks like connectcreds already has the functionality needed:
https://github.com/posit-dev/connectcreds/blob/53a4db92eb7b5fc81ed49118f662d88e7ff4886a/R/service-accounts.R#L76
https://github.com/posit-dev/connectcreds/blob/53a4db92eb7b5fc81ed49118f662d88e7ff4886a/R/service-accounts.R#L21