Support configuring the OLAP connector in `rill.yaml` and with `rill env configure`

[begelundmuller]

High-level changes:

• You can now configure the OLAP connector in rill.yaml instead of using the --db-driver flag:

# rill.yaml
olap_connector: clickhouse

• When running rill env configure, you will be prompted to provide credentials for the OLAP connector (unless it is DuckDB):

$ rill env configure --project
Finish deploying your project by providing access to the connectors. Rill requires credentials for the following connectors:

 - druid (used by my_dashboard)

Connector "druid" requires credentials.

? connector.druid.dsn : ...

Detailed changes:

• Adds support for setting an olap_connector: <name> in rill.yaml. If set, it takes precedence over the driver that is set using --db or --prod-db-driver.
• Deprecates the rill start --db ... --db-driver ... and rill deploy --prod-db-driver ... --prod-db-dsn ... flags (but keeping them for backwards compatibility). Moving forward:
  • The olap_connector key in rill.yaml can be used to set the driver
  • The DSN can be set using variables (e.g. --var connector.druid.dsn=...)
• Updates rill env configure to also prompt for credentials for the OLAP connector (unless it is DuckDB)
• Adds a new DuckDB config key connector.duckdb.path as a more DuckDB-native alternative to connector.duckdb.dsn
• Renames DeleteInstance.drop_db to drop_olap and moves the default behavior of only dropping for DuckDB to the runtime. This means it won't accidentally be set to true if deleting an instance that has set olap_connector through rill.yaml.