Add ability to designate a function for DB setup & automatically run it

Nobody has claimed this yet.

Slightly inconvenient that we have to call appropriate DB setup functions when using from the REPL, e.g. in Metabase:

```clojure
(metabase.db/setup-db-if-needed!)
```

It would be more convenient if we could tag a certain function as the DB setup function:

```clojure
(defn ^:toucan/setup-fn setup-db-if-needed! []
...)
```

and have Toucan automatically find and run this function using the namespace tools if the DB is not yet set up.

Start by locating the existing setup-db-if-needed! function and the namespace tools used for discovery. Define how a ^:toucan/setup-fn var is found and invoked when the database is not set up, while preserving current behavior; done means the REPL no longer requires an explicit setup call and the behavior is covered by tests.

Written by the indexing model from the issue text.