automatically configure spark on apple

[simonpcouch]

Some machinery to close #651.

Opening as draft as I've yet to integrate this into any functionality yet. Would configure_spark() be best situated as:

1. An exit handler on dbConnect<OdbcDriver>() (or dbConnect<DatabricksOdbcDriver>()?) that greps for an error message like the one noted in https://solutions.posit.co/connections/db/databases/databricks/#troubleshooting-apple-macos-users and a) automatically runs the configure_spark() function b) points the user to configure_spark() in a rethrown error
2. A helper that's always run inside of one of those dbConnect() methods
3. Something else?

I also imagine some sort of argument or env var that users could set to override configure_unixodbc_spark(unixodbc_install) and configure_unixodbc_spark(spark_config), but waited to implement pending a decision on the above.

Moving on to tests for now.