Consider init-fn for next.jdbc.connection/component

[seancorfield]

Is your feature request related to a problem? Please describe.
It's quite common that folks might want a specific function to run against the database when the connection pool is first created/started. Provide a way to supply that.

Describe the solution you'd like
An optional argument to component that specifies a function that will be called at startup on the pooled datasource.

Describe alternatives you've considered
Writing your own Component to wrap ->pool and call init-fn directly if supplied.